How does the ""&z="" function work?

I found a post on here that recommended using "&z="&Rurl() at the end of a GenAddRecord formula in order to, upon saving the opened form, redirect the user back to where they started from. This works great, but I would like to understand what's going on, because these functions seem useful in a variety of situations. 

So the way I think it works is that "&z=" tells the form where to go after the save button is clicked. Does that mean Rurl() is just the url of the record, and I could put any url in there to customize this function? 

Thanks