Starter solenoid

Its primary function is as the actuating coil of a contactor (a relay designed for large electric currents) which connects the battery to the starter motor proper.

When the ignition switch is turned on, a small electric current is sent through the starter solenoid.

When the solenoid is energized, it operates a plunger or lever which forces the pinion into mesh with the ring gear.

The pinion incorporates a one way clutch so that when the engine starts and runs it will not attempt to drive the starter motor at excessive RPM.

[2] If a starter solenoid receives insufficient power from the battery, it will fail to start the motor, and may produce a rapid clicking sound.

The starter solenoid is above the starter motor. The threaded terminal at left would connect to the battery through a heavy cable. At the right end of the solenoid coil, a linkage inside the housing would engage the pinion visible in the housing at the right side of the motor.
1: Main Housing (yoke) 2: Overrunning clutch 3: Armature 4: Field coils 5: Brushes 6: Solenoid