It's a term originally coined by Greg Raymer, I believe.
Very quickly (because I am tired and need sleep), a 'Stop and Go' is calling a preflop raise with the intention of pushing any flop, with a few exceptions. There are a couple of conditions for it to be feasible
- You must have a pretty short stack (obviously), but a big enough stack to give you fold equity after the flop.
- You must be out of position, and first to act after the flop, preferably into one opponent (the BB is the perfect spot to try this, of course).
Say you have AK in the BB. Button raises for about a third of your stack, everyone else has folded. You can either push there and then, or call, see a flop which your opponent is more likely to miss than hit, and push as the first to act, unless an A or K flops, in which case it would be often wise to let your opponent bet for you. It's in this sort of situation that the Stop and Go is marginally more +EV than a preflop push, because you are more likely to take the pot uncontested.
So basically, "Stop" = Call (preflop) and "Go" = Push (on the flop).