I am creating an Android application where users enter in their GPA, SAT, and ACT scores and select majors that they are interested in. The output shows a list of colleges in descending order of how closely their data matches that of the colleges (the best matching college is first and the worst matching college is last). I was wondering where and how in the app should I explain what the app does and what the output means? I wouldn't want users to be confused as to what the app does but also worry that just having text on the main screen explaining everything wouldn't look very nice.
I was wondering where and how in the app should I explain what the app does
You put that in the description of the app store, not the app itself. If the user has installed the app they know what it does.
The Facebook app doesn't have a "what's Facebook" section, or a "what do you use Facebook for" section.
and what the output means?
Ideally, you should not have to. The data should speak for itself and the meaning should be clearly discoverable.
If you do feel the need to explain how to read the data, a popular way to provide help in a mobile space is a tutorial overlay:
By providing an example set of static data you can point out what it means and how to read it.