It stands for Continuous Positive Airway Pressure. We wear a mask at night, attached to a machine that blows air to us under pressure to keep open our airway so that we don't stop breathing during sleep.
Some of us have a small mask that just fits over the nose, and others have a larger mask that covers the nose and mouth. Neither are pretty, but getting a good nights sleep makes a difference, besides which, if left untreated, it puts more of a strain on our hearts.