the torque converter takes the place of a clutch, allows the car to not stall at stops and is basically a torque multiplier that has two haves that slip and one half spins at a faster speed than the output shaft of your transmission. the lock up is when the torque converter 'locks up' and stops the slipping. it's actually sort of like a jet turbine hooked onto your transmission, but powered by fluid instead of the force of air. www.howstuffworks.com probably has a really good tutorial on it .