SEO (Search Engine Optimization) is search engine optimization, i.e. a set of methods and techniques aimed at improving the visibility of a website in search engines (e.g. Google).

What does it mean that a page is indexed?

An indexed page is a page that a search engine has found, checked, and added to its database (index).
This is important because only indexed pages can appear in search results.
If a page is not indexed, users cannot find it through Google or other search engines.