Putting a fork in something is to say, "Okay, we like where it is now, but we can take it in a different direction." This can happen as many times as there are developers out there. Usually it only happens a handful of times, though.
Imagine water running down a board, water is the software and the board is just there for you to visualize. Now imagine you put a fork in the stream somewhere. Different streams after the fork, but at the base (before the fork) they remain the same. Crap analogy, but it works for what you need it to.