Web Search Optimisation, often called SEO, means improving a website so search engines can understand its pages and people can find useful answers there. It focuses on unpaid search results. You are improving the website itself, rather than buying an advertisement.
For example, a business offering bathroom renovations needs a page that clearly explains its service, the areas it covers and how a customer can enquire. Search work helps organise that information, check that the page can be accessed and connect it with other relevant pages. There is no need to fill the writing with repeated keywords.
The aim is relevant visibility. A visitor looking for something you actually offer is more useful than a large number of visitors looking for something else.