The bootlid tailgate release cable is a cable which is attached to a lever or button in the front of your vehicle. The cable attaches to the boot release mechanism in order to open the boot without the driver having to leave the vehicle and which is powered by an attached motor. 

Through general wear and tear the bootlid tailgate release cable can stretch and eventually snap. 

If it fails, you will not likely be able to open the boot from inside the vehicle and will have to use the key to lock and unlock the boot.  For some vehicles, however it’s possible that you will not be able to unlock the boot at all. Some failed bootlid tailgate release cables will trigger the vehicle’s alarm system, believing that the boot is unlocked.  Some models have an emergency lever in the boot to release the boot lid. 

It is also worth noting that the boot release can fail and the cable may not be affected.  A good way to check is to listen for the electrical solenoid which will make a small noise when operated.  If you cannot hear this your problem may be related to the electrics rather than the cable.