A starter motor is normally not connected to the engine. When you want to start, you apply power to the 'starter motor solenoid' a high current low resistance switch that connects the positive battery to the starter motor with a large conductor, using a metal ground as return (to which the minus power is connected). The motor turns and the motion connects a gear to the engine flywheel and begins to turn the engine as spark plug power and other power is also connected. Some times the starter solenoid gets stuck and will not close the high current switch = click, click, click. Some times a whack can release the solenoid, but if the battery is very low - click click = no hope whacking it!. If the motor starts, the driver stops turning starter power switch and the starter stops and the starter gear is spring disengaged, and the motor (hopefully) operates. If not, some actions can help if the motor has gasoline/diesel and other normal functions and a limited number of re-starts can be tried, before the stored power in the battery is gone. After that, call AAA, get a start/tow....
The brushes wear down. The springs atop them can need a little help: tap! Or, the solenoid can stick as its lubrication fails.