The code says, when the page loads, run the init function. The init function sets an event based function, which says when someone presses a key, this is the function.
For keyboard based events, there's a keyCode property associated with the event, which is where e.keyCode comes from.
I'm not sure if it's different for Mac/Linux machines, but on Windows, 13 is the key code for the Enter button.
So in all, it says "when a user presses a key, determine what key they pressed. if they pressed the Enter key, go to a location. In this case, we're reloading so, self.location works.
I just put this code together real quick and tested in firefox 1.0 and IE6. Might not be fully cross-platform/cross-browser, but it's a starting point.