check solenoid
There could be a bad sylanoid or a bad starter...
Check the battery cables or starter.
Dead battery? Loose or dirty battery cables? Bad starter solenoid? Bad starter? Neutral switch?
If the battery tests good and battery cables are clean and tight at both ends and you are jumping the starter solenoid correctly with no results, you probably have a bad starter/solenoid.
Sounds like starter solenoid needs to be replaced
Voltage Regulator?
Double check the battery cables at the battery and at the engine, be sure they are clean and tight.
may be on top of the starter and replaced with same. Follow the + battery cable it will connect to the solenoid
check your alternator or make sure the starter is on correctly
what does it do
You need to replaced the starter or the alternator.
Yes, by a shop that rebuilds starters.