What is theta in a circle?

Could you possibly explain to me what theta represents in the context of a circle? I've been trying to wrap my head around it, but it seems like a rather elusive concept. Is it related to the circumference or the area perhaps? Or does it have something to do with the angles within the circle? Could you break it down for me in a way that's easy to understand? I'd really appreciate your help in clarifying this matter.