Your uncle is your parent's brother, or the man who marries one of your parent's siblings. Your favorite uncle might teach you to play chess or take you to amusement parks.
Usually, an uncle is a relative — the brother of your mother, for example, or your dad's step-brother. It's also common in many cultures to have honorary uncles who are close family friends. From your uncle's perspective, you are either his niece or nephew. Colloquially, when you say or cry uncle, you're admitting defeat: "I'll let go of your arm when you say uncle!"