Clutch is responsible for disconnecting the engine from the gear box by the use of a pedal. In cars with an automatic gear box, the function of clutch is performed by torque converter (fluid coupling). The torque converter works automatically depending on the engine RPM.
Issues with the car clutch can be suspected if the changing of gears is not smooth or it is impossible to change gears at all. Unfamiliar noises or a smell of something burning can also indiacte problems with the clutch.