Torque converter is what slides over the shaft on the trans and bolts to the flex plate. Torque converters are a bit hard to describe as to how they work but basiclly they multiply the torque of the motor to get the car moving.

The "stall speed" of a converter is a rating for the RPM that the car will launch at if the pedal is floored more or less...also called "flash" by some people.

Example: if you have a 8" converter that has a stall speed of say 5500 rpm and your sitting at a dead stop and floor it the motor will jump to 5500 rpm before the power will nail the rear tires. (kinda like poppin the clutch in a manual trans car).

BG