Limiting Frames?

I'm kinda confused on how to limit frames in my game, I was looking at some code when I came accross this:

Timer = MilliSecs()
...
...
.Main
If MilliSecs() > Timer + 1 Then Goto Render Else Goto Main

Can someone explain to me how this limits the frames produced? I can use it but I prefer to know how it works first.

http://www.blitzbasic.com/Community/posts.php?topic=77410

Thanks Gabriel, I have alot of fun reading to do now.