A cough machine (also called a mechanical insufflator-exsufflator) is the device you're likely thinking of. The most well-known brand name is the CoughAssist (made by Philips Respironics).

Other names it goes by:

It works by delivering a positive pressure breath (insufflation) followed by a rapid negative pressure (exsufflation) to simulate a natural cough and clear airway secretions. It is used in patients with neuromuscular diseases (ALS, SMA, muscular dystrophy, spinal cord injury) who have weak cough function.

If you meant a different type of device - such as a CPAP mask, a nebulizer mask, or something used in a cough testing/screening context - let me know and I can clarify further.