A dust bunny is a big ball of dust and other tiny things that collect together under things like the bed, couch or other furniture. It's like when you don't clean your room for a long time and all the dirt and hair and little bits of other stuff come together and make a big clump. The more stuff there is on the floor, the bigger the dust bunny can get because it has more things to stick to. So, it's a big ball of dirt that you can sometimes see when you lift up the furniture or clean your room.