The torque converter adopts the role of the clutch in conventional automatic transmissions and transmits the forces between the engine and the transmission. Torque converters usually operate hydrodynamically, ie, they transmit the drive moment of the motor by the flow of a liquid, usually oil.
The torque converter works especially when starting (engine rotates, vehicle stands) by its ability to transmit torque and speed infinitely.
The DSG automatic dual clutch transmissions do not require a torque converter. They have couplings for power transmission.