Cat Behavior

What does it mean when my cat’s ears are back? He (Peter) was lying on the coffee table in the middle of the living room right where my dad and I are were doing our own quiet activity. It’s not noisy and the temperature is cool. He’s quite vocal and will fight with our other cat, Bea, who’s one month older than him. She won’t usually fight him back when he gets aggressive with her and they always make up with each other.

We adopted them at the same time when they were kittens (they’re now 8) because they were the only two cats in their room at the shelter who played with/got along with each other. They both get equal amounts of attention from my siblings and parents. The only time Peter’s ears are back is when he hears a noise he doesn’t like, the same goes for Bea. What does this behavior mean?