A list of index terms used by a specialized database is known as a thesaurus. The terms in the thesaurus are organized into a tree structure.

This example comes from ERIC. You can search broadly for INSTRUCTIONAL MATERIALS, or you can choose a narrower type of instructional material, such as PROBLEM SETS or TEXTBOOKS.

When you want to search every article that has been tagged with a term or with any of the terms beneath it, this is called "exploding" your search term. When you "explode" a term in some databases, every term visible beneath it is added to your search as well. 

Because some terms in EMTREE, MeSH, or CINAHL can have hundreds of narrower terms, most often a database platform will not automatically add every search term beneath your term to your search string. It is still searching all of those terms as well, though.