Subject terms are a form of descriptive metadata. At their simplest they may be tags chosen by the authors, but most databases use a controlled vocabulary assigned by professional catalogers. Subject terms may also be called index terms, MeSH, subjects, subject headings, etc...
The advantage of subject terms is that they're standardized terms which will be assigned to all content covering that topic. So no matter what terminology or language is used by the author, the same subject term will be used.
For example, Academic Search Ultimate uses the subject term "Gender Wage Gap". Again, even if the article's author uses the keywords pay inequality or wage gap or gender pay gap or equal pay, a subject term search of Gender Wage Gap will retrieve all of the articles on that topic.
Whenever you find a good article in a database, check out the subject terms. If one or more of them look like matches for your topic, re-run your search using those subject terms--and be sure to specify you want those terms in the subject field. That will ensure the search results are really about that subject and don't just contain that word in the record (but aren't really about it).
One thing to watch out for: Each database has its own controlled vocabulary for subjects. This means that if you do use subject searches, you'll need to discover the subject term for your topic in each of the separate databases.
Subject Term for Gender Wage Gap
|Gender Studies Database
|Academic Search Ultimate
|SOCIndex with Full Text
As you can see, the usefulness of the subject headings can vary, particularly if you're searching across multiple databases. And remember that EBSCO is not a database. It is online publishing companies that hosts multiple databases. So even though you can search multiple EBSCO databases from the same search box, the subject terms won't be consistent.