Open Question: can someone explain this driver's insurance deal to me?

I don't have a license yet because I need to have proof of insurance to drive the car at the DMV. However, if my parents' cars are already registered to insurance, then I don't need to join their insurance or something and i can just drive their car right? My dad said if i "join" our family's insurance, the rates will get higher. But i don't get this. I thought it was just the car that