When someone wants to buy something, find a restaurant, whatevs, they often start with a search engine, usually Google. They'll start working through the list of entries. Entries near the top of the list will get more clicks, and more visits.
Search engine optimization (SEO) is about giving your site a good chance of being high in that list of search results. SEO is a Big Deal. It can make the difference between success and failure for a small business website.
You can rank (SEO talk for getting high on the list) by giving Google money. You don't have to, though. The way you write your content, structure your site, and other things you can do, affects your ranking. That's called organic SEO (though I'm not sure why). We'll talk about organic SEO, rather than paid SEO.
Free is good.
We'll focus on Google, simply because it's the most used search engine. The same techniques apply to Bing, Baidu, and other search engines.
These lessons draw from a course by Yoast, a company selling WordPress SEO plugins. Consider using their plugins if you run a WordPress site.