Theft Protection (TP) is a type of cover for hire cars. It limits the driver’s responsibility if the rental car is stolen. This means that the car hire company will not charge you the whole cost of the car if it gets stolen while you have it.
There is almost always an excess, which means you will pay the first part of any repair or replacement costs.
The specifics of what Theft Protection includes depends on the car hire company and where you hire the car.
Also, Theft Protection only works if you stick to the terms of the rental agreement (which you’ll sign when you collect the car). This means that if (for example), you leave the car unlocked and it is stolen, you will be liable for the whole cost of repair or replacement, not just an excess.