Think of subject terms and headings as hashtags that can point you to more sources with the same tags.
When you find a useful book in Melvyl, look at the record's Library of Congress Subject Headings, which are hyperlinked in blue. Click on one to discover other similarly tagged items or use the headings to give you ideas for terms you can try in a keyword search.
Here are the subject headings for the book Pushout.
Most EBSCO databases have a tool to help you discover subject terms. See Academic Search Complete > Subject Terms (find this at the top)
Most ProQuest databases have a tool to help you discover subject terms: See PsycInfo > Thesaurus (find this at the top)