Many (but not all) databases allow subject searching. A subject search is more specific than a keyword search: it looks in only one field of each record - the subject field.
Subject searching uses specific terms, known as subject headings, that describe the content of the document (book, article, video, map, etc.). The subject headings come from a predetermined list of possible terms known as the database's thesaurus. The linking between the subject headings and the documents in the database is known as the subject indexing.
Many databases use subject headings that are unique to that particular database. This controlled vocabulary allows for consistency of subject terms across the database. Most databases that use subject indexing provide a way to search the thesaurus. In the thesaurus, subjects are often listed with broader, narrower, or related subjects. Using the database's thesaurus will help you identify most effective search terms.
When researching topics within a special sphere of knowledge like science or medicine you may find better, more detailed information by searching databases that support those specific fields. You can visit the A-Z Databases page and use the Subject filter to find relevant databases.