The loop expression takes keyed animation and does something immediately after.
It'll either PING PONG, (go back and forth with the animation)
CYCLE, (which loops the animation from the beginning over and over again)
or CONTINUE (which keeps the value of the animation going in that direction)
loopOut("one of the 3 options"),
Personally, I almost always use loopOut("[one of the 3 options]"). What I'll do is key an animation for a property at the start of the timeline, then apply this expression. Easy peasy. I might use loopOut("continue") for linear actions, lets say for a rotation or loopOut("cycle") random animation that I want to keep going forever.
I learned a little more about the loop expression today. This blog post really explains this part really well. You can also determine how many keyframes you want to LOOP with the second value of the expression.
Shinsuke's animation shows really well what this means. Also check out the Adobe help page about this expression and more.