What The Hell Are Master Rights?

Master rights – What the hell are they? Several times I’ve seen some form of this question come up on Facebook, and the replies people get are usually hazy, conflated or inaccurate. So here I’m going to attempt to explain this as simply and completely as I can, and at the end, I’ll link you to some other very good explanations and resources. Be warned, even this is super simplified, but I hope it helps.