A humidifier is a part of a CPAP machine that adds moisture to the air you breathe via a small heated tank of water. The humidified air helps to prevent a dry mouth during the night. Some CPAP machines may have humidifiers built-in, while others may use a separate part.
So... how does it work?
The bottom of your humidifier chamber has a metal plate that the machine heats up during the night. When the chamber is filled with water, the heat creates water vapor. The air that travels through the machine passes through the humidifier and collects the moisture and transports it up the tube.
Keep in mind that it is important to make sure that your humidifier and tube are cleaned regularly with warm, soapy water.