Jeep does not start but makes a fast clicking sound. What is it?

Clicking sound is the starter solenoid again and again pushing the starter motor armature forward against the toothed outer edge of the flywheel. Check the heavy cables and connections from the battery to the starter motor and the earth strap from the engine back to the frame. If everything is OK then remove the motor and examine the electrical striker plates inside the motor body...you will have to open the motor up on the bench. The striker plates are copper and when the solenoid pushes the armature the contacts hit the striker plates allowing heavy current to be drawn by the starter motor to crank the engine over. These striker plates wear out. Fortunately there are cheap kits that allow you to replace the striker plates. Once fixed the motor should last a good number of years. One pece of advice, clean the motor interior if you feel you have to but do not lubricate anything, it only attracts the dirt and this will kill the motor in short time.