PEP stands for Positive Expiratory Pressure.
PEP involves breathing out against resistance, a bit like breathing through a straw.
The air that is waiting to be exhaled creates pressure in the lungs which helps splint, or hold, the airways open.
This gets air behind the mucus sitting in the airways to help move it towards the mouth so it can be coughed up and cleared.
Resistance can be provided in many different ways, using water, a mouthpiece or a mask.