Broadly speaking, academic sources are books and articles written by academic experts for an audience of other academics. As college students in a 300-level Spanish literature course, you are among the audience for academic sources.
Many, though not all, academic sources are peer-reviewed. When an academic article is submitted to a journal for peer-review, other experts (peers) in the same field must read (review) the article for originality and quality of research before it may be published. Not every article that goes through peer-review is actually approved for publication, and many that are published have to undergo revision--sometimes substantial--first.
Several of the databases on this guide will allow you to limit your results to peer-reviewed publications.