Taxonomies in WordPress are most commonly seen as Post Categories and Tags. You can also register your own taxonomies, and many plugins use their own Taxonomies to facilitate a lot of functionality.
Bonus! Using Term Archive Priority you can have your taxonomy term archive pages be returned in search results!
Taxonomy terms (e.g. your Categories/Tags themselves) can contain content that’s very relevant to search! WordPress native search doesn’t consider taxonomy terms when performing searches but SearchWP indexes and considers that content.