When you do a search on a search engine, the search engine algorithm attempts to measure how relevant pages in its index are to your search phrase and it displays the most relevant pages at the top.

Improving the relevancy of a page for a particular search term or search phrase is an important part of SEO. It’s important to note that the SEO’s job is not to make the page more relevant (the content does that), but rather to make sure that the search engine can accurately determine its relevancy.