Fountain of Knowledge

In all of my research, I've never seen this spelled out anywhere, and I figure I'm just not looking in the right place. My question is about book advances. The name, to me, implies that this is an advance (duh) toward what your book sales will do.

So, combining that with royalties - does that mean that your royalties have to reach and exceed your advance amount before you start getting those 'regular' royalty checks? Or is the advance in addition to, and completely separate from, any royalties earned?