Search engines like Google and Bing are the librarians of the Internet, their systems collect information about every page on the web, so they can help people find exactly what they're looking for, and every search engine has a secret recipe called an algorithm for turning all that information into useful search results. Now, if you own a website, search results matter. When your pages have higher rankings they help more people find you, the key to higher rankings is making sure your website has the ingredients search engines need for their recipes, this is called Search Engine Optimization or SEO.
As it turns out, most of the big ingredients are known, let's take a look.
First, words matter!
Search engines account for every word on the web, this way when someone searches for "what is SEO" the search engines can narrow your results to only the pages that are about those words.
Second, titles matters.
Each page on the web has an official title, you may not ever see it because it's in the code but search engines pay a lot of attention to page titles, because they often summarize the page, like a book's title.
Third, links between web sites matter.
When one web page links to another it's usually a recommendation telling readers this site has good information, a web page with a lot of links coming to it can look good to search engines, but some people try to fool the search engines by creating or buying bogus links all over the web that point to their own website. Usually search engines can detect when a site has a lot of them and the account for it by giving links from trustworthy sites more weight in the recipe.
Fourth, the words that are used in links matter too.
If your link says "letsforum is a SEO forum" and the word SEO is linked, search engines can establish that letsforum.com is related to the word SEO, this way when someone searches for SEO that site will rank well.
Lastly, search engines care about reputation.
Site with a consistent record of fresh engaging content and growing numbers of quality backlinks, may be considered a rising stars and do well on search rankings.
These are just the basics, and the recipes are refined and changed all the time, good SEO is about making sure your website has great content that supported by the ingredients that search engines need for their recipes.