The torque converter slips hydraulically similar to a clutch and locks up at predetermined RPM which is called stall speed. Higher stall means that, when compared to a stock torque converter, the higher stall converter will "lock up" at a higher RPM so the engine rpm more quickly reaches it's best torque and/ or horsepower range sooner. This tends to offer faster acceleration and performance, but reduces fuel mileage and stress on the tranny in the form of heat.
Have fun, Adam Bush