Ah! They're the weaknesses of the enemies. I might need to add an NPC that teaches you that.
lol. It's funny, immediately after I asked the question, I wondered if that had something to do with it. Regardless, I think it's a nice mechanic in the game, even if I didn't fully understand it at first. I think you're right though. I may just may have been blinded by how great and sexy the game is overall, but it wasn't readily obvious to me. An NPC or even a sign or note might help out the oblivious like me.