Very stupid code question regarding millis()

By reference I mean something the Arduino can compare something else to. If not a reference, what is it for?

From what I can see, the Arduino runs the gesture through the if and returns a value of either true or false. What I'm trying to figure out is what would be considered "true" coming out of the loop.

EDIT - Wait, it looks like it actually sends out multiple values. I'm less confused, but I still don't know what it does.