In its most positive form, SEO is those techniques a webmaster can use to help the search engine understand the web site. When a webmaster operates with high integrity, they are interested in serving the public by being clear about the content of their web site, so the search engines can understand that content, so that the search engines can closely match their site with the readers who are most likely to value their site.
Unfortunately the SEO field is full of junk techniques for making junk websites that play off bugs in the search engine algorithms. I, of course, will not cover those techniques here.