A machine that fills the air around it with moisture is called a humidifier. Turning on a humidifier can help you breathe more easily when you have a bad cold.
When the air in a room is too dry, it can cause all kinds of problems: itchy skin, irritated eyes, and coughing. A humidifier is a device that humidifies (or adds moisture to) a space by sending steam or water vapor into the air. The word comes from humid, or "damp," and its Latin root humidus, which means "wet." If your houseplants are turning brown and slightly crispy, you should definitely try using a humidifier.