The clutch is a device for disconnecting and connecting rotating shafts. In a manual gearbox, the driver pushes down the clutch when changing gear to disconnect the engine from the gearbox.
There are several parts in the clutch, that can become worn or jump, the discs can be worn, as well as a spring system either breaks or becomes slack. All these parts can be replaced independently of each other. However when changing the flywheel the entire clutch needs to be changed. This can be an expensive job.