What is a solenoid
A starter solenoid is the part of an automobile which switches a large electric current to the starter motor, in response to a small control current, and which in turn sets the engine in motion. Its function is identical to that of a transistor, but using an electromagnetic solenoid rather than semiconductor to perform the switching. In many vehicles the solenoid also engages the starter pinion with the ring gear of the engine. A solenoid is generally just a coil of wire that's wrapped around a hollow non-magnetic core. The solenoid receives an electric current from both the ignition switch as well as the battery. Solenoid problems are a very often hidden problem that can cause quite a bit of electrical problems. Generally when a solenoid gets broken the two metal clamps won't operate anymore to complete the circuit, making it impossible for power to flow from the battery.  If your car won't start and you've changed your battery or jumped it, and if there's no sound at all out of the starter when you turn the key, it's a good sign that your solenoid is bad.

When a solenoid is broken, it no longer is able to connect the battery to the starter. A solenoid can break through a variety of factors; one is just wear and tear or, poor technique in jump starting a car. If you've gotten a faulty solenoid and you want to replace it, it might be a simple job unless it’s buried under a lot of components.  If you go into your local auto parts store and find the solenoid you need for your car, they should be able to give you some instructions on how to install that particular solenoid into your vehicle. Make sure you disconnect your battery first so that you don't accidentally shock yourself.