Many databases have a thesaurus where you can find all the keywords used in that database (see also keywords). You can search here for search terms to supplement your query with. Especially subject-specific databases often have a good thesaurus to help you find search terms.
Example
Suppose you are looking for literature on English language education. Since most of the results you find are about learning English in English-speaking countries, you decide to consult the thesaurus of the educational database ERIC. That one gives you the following suggestions:
If you then click through on one of the suggestions, you are usually shown broader, narrower and related keywords. This functionality is therefore the main difference between a thesaurus and a regular keyword list; in a thesaurus, the keywords have a so-called hierarchical relationship with each other. If you are still in the orientation phase or are in the process of coming up with search terms, this can make a thesaurus especially useful.
If you want to know more about working with the thesaurus, check the help section of the database you want to work with.