A taxonomy within WordPress is a way of grouping posts together based on a select number of relationships. By default, a standard post will have two taxonomy types called Categories and Tags which are a handy way of ensuring related content on your website is easy for visitors to find. These two types of taxonomies are included in WordPress by default, but just like any other taxonomy, can be removed or changed and you can even add more if you like.  (official doc)

link of official doc