What does a lawnmower solenoid do?

You don't NEED it exactly its just a fancy box that connect your battery wires to the starter. If your lawnmower is clicking/wont start or silent when you turn the key its possible its the solenoid. undo the rubber covers from the solenoid touch the two bolts together using something metal and start it up, bam. you just by-passed the solenoid. (the solenoid will be next to the starter in most cases and have two rubber wire covers on them)