A sock is a piece of clothing you wear on your foot, under your shoe, and socks usually come in pairs. You can wear only one sock if you like, but your other foot might get jealous.
Some socks barely cover the foot, while others reach up over the knee. When you sock someone, it has nothing to do with footwear; you punch them. If something "knocks your socks off," it makes you extremely enthusiastic or excited, and if your brother tells you to "put a sock in it," he means "be quiet."