The flywheel is a heavy plate fitted to the end of the crankshaft which smoothes out the engine's power delivery and acts as the input for the clutch.
The flywheel's mass stores and releases kinetic energy as the engine rotates, helping to damp out fluctuations in engine speed caused by the power strokes in the cylinders.
The flywheel also transmits power from the engine to the clutch, which in turn takes power to the transmission.