The bonnet pull cable releases the latch mechanism on the bonnet.

The bonnet is a crucial part of the car body, for beneath it is housed some of the most important components a car needs in order to run. In most cars the bonnet is the hinged cover that protects and allows access to the engine compartment. A bonnet is made up of an outer panel which is designed to be smooth and streamlined, assisting with the aerodynamics of the vehicle, contoured to match the shape of the car, and an inner panel which is strong and offers protection to the engine beneath. 

The bonnet is fixed to the frame at the front of the car and, with the action of hinges, opens outwards.  The bonnet works with a bonnet latch mechanism. It is held down by a latch which stops it from opening unexpectedly while driving along and also acts as a lock. The bonnet can be opened from the exterior, by releasing the latch, hidden just under the bonnet lip, or by pulling a small lever inside the car, usually located beneath the steering wheel, which is attached to the bonnet pull cable, which causes the bonnet to pop open.

If the bonnet pull cable has worn thin and slack, stretched or snapped, the release mechanism won't function correctly and it will need to be replaced with a bonnet pull cable that will ensure it is compatible with the bonnet.