In the simplest of terms, a torque converter is a fluid coupling between the motor and transmission. It takes the place of the clutch in a manual transmission set up. It has two sets of turbine vanes, the first set is bolted to the motor side, the second set is on the transmission side. When the motor side turns, the vanes push fluid against the transmission side. When enough fluid pressure is achieved, the fluid turns the transmission side and moves the car. This allows the car to idle in gear without the car moving or the engine dying.

Stall speed is the point where the Engine 'Stalls" when you put one foot on the brake and the other hard on the gas. Most stock converters stall between 1200-1500 rpm. A high Stall converter is looser and allows the motor to rev to a higher RPM before it hooks up and starts pushing the transmission side. There are numerous Stall settings that can be built into a converter depending on planned usage.

The down sides of higher stall speeds are higher heat and less efficiency at higher RPM. The slippage generates heat and also does not lock up as tightly. To deal with this last issue they now make "Lock-up" converters that actually mechanically lock up at a given RPm or condition giving near 100% efficiency. This is usually controlled electronically

Hope that helps you understand

Pat