The Library of Congress is the authority that controls the vocabulary of subject headings that academic libraries use to organize their collections. Understanding and knowing subject headings can open up many resources that might not be found through keyword searching.
Subject heading examples:
The tricky part is that these subject headings are controlled, meaning that the Library of Congress (LOC) decides what they are and then libraries have to use them. So it's helpful to know where to find these subject headings.
Once you've got some subject headings, you can search using them in the library's catalog.
Now it's just a matter of exploring!