What is wrong with your car when it clicks when you try to jump start it?

"Clicking" when trying to crank an engine

Usually this indicates that the battery is low, BUT... since you question indicates this is happening while trying to jump start it with [HOPEFULLY] a FULLY CHARGED battery, then the problem is elsewhere.

The "clicking" sound is created by the starter solenoid "cycling." The solenoid has a spring to hold the contact points OPEN, and when you turn the key to the starter position, an electric current from the battery is supposed to pull the contacts CLOSED [against the spring] causing current to flow to the starter, thus causing it to turn.

The clicking of the solenoid means that there is not enough electrical current power to HOLD against the return spring, so the spring pulls the contacts open, and the battery tries to close them.

The cause of the reduced power in the solenoid must be found, identified, and corrected.

Although there could be several causes, these are the ones that come to mind at the moment:

• One of the connection between the battery and the solenoid is LOOSE, or
• One of these connections has been degraded by CORROSION within the connection,
• One of the cables or wires in that part of the electrical system has been reduced in size by being partially broken or cut, allowing some, but not enough current to flow, or
• The solenoid has become defective internally.

I repeat, these are the most common causes that come to mind, and thus there may be something else.

NO ONE can tell you the specific cause without personally doing a "hands on troubleshooting" of the starter circuit of your vehicle.