SEO is an acronym for Search Engine Optimisation. It simply means indicating the search engines that the pages of your website are worth being shown in the result pages (SERP). In addition to optimising the pages of your website to appear in search results, there are also strategies and techniques that are used to improve the chance people will click the links and stay on the website after clicking the link.
Search engines are such systems as Google, Bing and others, which are used for searching for information, images, videos and more on web. When a person makes a web search, they are given a list of results. The search engines attempt to show the most relevant results for the person who makes the search.
In order to show the most relevant results, search engines use multiple different signals or factors to rank your website and its contents. The search engines use algorithms, sequences of calculations, to give your website a score. This score is used to put websites into a ranking order, which is used to determine if a page should be shown in search results and how high in results it should appear. The higher score your website has, the better chance it has to show high in SERPs of searches people make.
SEO isn't exact science. There are multiple distinct search engines, which are updated frequently. Google is known to use at least 200 factors to rank your website's worthiness, content quality being one of the most important signals. Thus the ranking factors changed quite often. Some of the ranking factors are more important and others less. You have control over some of them, while others you cannot really affect at all.
Amongst the ranking factors is the amount and quality of backlinks, the links from other websites directing to your site or its content. It means you should, in a way or another, get other websites to link to your site. However, not all the backlinks are equal as ranking signal. A website linking to your site which has a high search ranking is a better ranking signal than a website which has low search ranking.
The best kind of backlinks are organic links. Organic links are created without you asking (or paying) for them. They are the links people make to your website content from their blog posts and other content, preferably without having any affiliation to you or your business. They are the kind of links you really can't affect. The only thing you can do is to create the kind of quality content people will want to link to from their blog articles and other places.
Search engines like to show popular content in the SERPs. In addition to the amount of links from high quality sites, there are other factors that may be used as a signal of the popularity of your content. These factors include overall traffic of your site, how often the links to your site are clicked in search results and how long the person stays on your site after clicking a link (aka dwell time), and even the amount of non-spam comments in your blog posts and social media shares of your content. There are multiple theories of if and how these factors affect the search ranking of your website and its pages. I need to note here that despite the rumours, bounce rate is not a search ranking signal and will not affect your search visibility.
As mentioned above, quality content is one of the most important ranking factors. It means your website content, all the text a search engine can "read", is high quality and unique. In separate web stores that sell the same brands, the product information is often same or very similar. This causes the web store to lose quality points, because the content is not unique. This kind of content is know as thin content.
Some people claim that content length, amount of words, is used as a signal of quality. There are, however, many cases where very short content is shown higher in search results than longer content of the same subject. This suggests that word count isn't a quality signal, at least not alone.
For you have made web searches yourself, you know that search engines rely heavily on keywords. When you make a search you use certain words, often in a format of a phrase, of the subject you are searching. To more strategically optimise your content for search engines, you can use keyword planning tools which help you with choosing the most effective words and phrases to use in your content.
Since I get most of my website traffic through search engines, there's a high probability you ended up to this article through searching something SEO related. Making that search, you used some of the words or phrases that are included into this article or its title. A spider, a bot which is coded for this purpose, of the search engine you used has visited and crawled some or all of the pages of my site. It has found this article and went through it to find out which words and phrases are included in it. Then, depending on the overall ranking of my site as well as the ranking this article has received based on its quality and other factors, the words and phrases in the article are matched with the keywords you used in the search you made.
In addition to the more general ranking factors, search engines are increasingly personalising the search results for different people making the searches. The personalisation depends on multiple things, such as your location and language, the searches you've made, the websites you have visited, the links you have shared on social media, and other methods search engines use to determine your interests. That is why the SERP you see is not the same as the SERP I see, even if we use exactly the same keywords.
On top of trying to get your website to rank high and be shown in SERP, you must get people to actually click the link in search results. This can also be partially affected, or rather suggested, with SEO. First of all, the title of your content is usually the most prominently shown part of the search results. The other parts visible in search results include the content url (the address), the publication date and a short description snippet. The description snippet may be a part of the content, which includes the searched keywords, or it can be content of the content's meta description.
Optimising the title and other parts that are shown in search results, you can possibly get more people to click the link. It's important to note that while you can suggest the search engines to use a certain title or description for your content, the search engines may or may not follow those suggestions. Using a title which doesn't really fit the content of your page will only harm your website's search ranking.
Basically SEO is quite simple. The best way to optimise your website for search engines is to create quality content and make sure your site works well for the users. It doesn't matter at all if your content is in technical sense optimised perfectly for search engines, if no person ever visits your website. To optimise your website for search engines you need to create unique quality content, which people will want to read and share.