Stall speed is the engine RPM at which the torque converter transfers the power of the engine to the transmission, or, the amount of rpm the engine can reach with the brakes locked and transmission in gear before the drive wheels turn.
The stall speed assigned is simply a way to rate the converter’s performance level. The torque converter will list a range which tells you what to expect and at different levels you should be able to footbrake stall the converter to the level listed depending on the vehicle’s setup. High performance torque converters should provide an obvious improvement in the vehicles’s ability to accelerate from a dead stop.
Here’s another way to look at it. The torque converter’s stall speed roughly equates to the clutch engagement point on a manual transmission.