When we ask "what is this opera about?" what we're really asking is, "what's the subtext?"
Some operas have none, others are loaded with historical and cultural references that are important to be aware of as they can inform how you choose to intone and express certain lines in your part. While there are no opera cliff notes, there are resources. Several of them are listed below.
Not one synopsis, multiple synopses. Quickly reading a variety of synopses can reveal differences in historical interpretations of the opera as well as cultural differences between American, British, French, Italian, and German interpretations of the same opera.
Composer biographies often include fairly extensive information about the immediate context surrounding the creating of an opera - where was the composer living, who and what were the influencers in the composer's life at that time, what were the major personal, social, cultural, political events affecting the composer, etc.
Pro Tip - since time is a luxury at this point, only read the portions of the biography that relate specifically to your opera. Discover your target pages by going straight to the index at the back of the book and looking up your opera.