The Flywheel is a part of the vehicles engine.
The Flywheels purpose is to allow the engine to spin when the starters gears engage the flywheel.
The large diameter of the Flywheel allows the small starter motor to turn over the large engine which then allows the engine to start.
The Flywheel is bolted to the engine so that when the starter is operated then it spins the engine and without the flywheel the starter cannot turn the engine over to start it.