As my friend Marty Bergen says, he advises students to pray that reverses don't come up.
Basically, a "reverse" is when opener's rebid (2nd bid of the auction) meets both A & B:
A) Opener's rebid is in a HIGHER ranking suit than his first suit
B) Opener's rebid is at a HIGHER level than responder's response (but never a jump!)
Just those two simple lines are enough to make many of my students' eyes glaze over.
So, these are reverses: