The water reservoir (also referred to as a water collection tank or water tank) on your tumble dryer serves to collect the water that gets expelled by the appliance.
It's possible for this tank to end up getting blocked, obstructed or even broken over time. When this happens, the appliance could begin to leak, and you will probably find it won't dry properly.
Try cleaning the water reservoir out with vinegar and warm water. If it's broken or too badly damaged, replace it by ordering the right model to fit your machine.