Follow the link and you get a good description. The basic idea is that you can use win probabilities to figure out if it's a good idea to bunt, steal, hit and run, etc. It'd be really cool it you could just pick a live game and have it set up the situation, but the world is not that cool yet so you will have to enter the inning, outs, and baserunners.

H/T to ESPN Insider Rob Neyer.