Solenoid is the relay or in other words electrically activated switch that connects the 12 Volts from the battery to the electric starter motor. Your key switch sends 12 Volts to the solenoid which then intern sends 12 Volts to the starter motor. Sounds like the teeth on the gear of the starter has worn / ground away and are not meshing with the teeth on the flywheel.
Specifically what model engine does that have on it?
Specifically what model engine does that have on it?