When something's common, it's usual, or it happens frequently. It's more common than you might think for little kids to be terrified of clowns.
Things that are common happen all the time — they're ordinary. It's common for people to put ketchup on their French fries, and a pigeon is an extremely common type of bird to see in a city. Common can also refer to something that's shared by a group, like a region's common culture. In fact, a green space in the center of a town is sometimes called a common, maybe because it's shared by everyone in the town.