the simplest explanation (while not entirely correct) is that a stallie is like a clutchless manual. because an auto trans doesnt have a clutch to disengage the transmission from the engine is uses a converter.
the converter allows the car to come to a complete stop whilst the engine can still run (ie, not stall).
if you were to try and rev the engine at a complete stop you would have to press harder on the brakes so it wouldnt move, because as you rev more torque is passed from the converter, throught the trans and onto your back wheels.
a high stall converter (or a stallie) raises the stall point so less torque is distributed to the wheels when reved at under the stall point (ie, a 2500rpm stallie) and thus the braking force is much less than it would be if you did this on a standard converter, allowing you to "stall up" the converter so when brake pressure is released and further rpm (acceleration) is applied a much higher amount of torque is passed onto the wheels.
all of this sudden torque (instead of gradual torque with a lower stall point converter) allows for very quick take offs (and burnouts,lol). the best stall point for a stallie is just before you max torque point, and thats why most high stalls are around the 2200-3000rpm mark (and even more for serious drag beasts).
hope that helps your understanding, it is all a bit confusing at first, but once you get your head around it, it makes alot of sence.