To humidify means to add moisture or water vapor into the air, thereby increasing its relative humidity. This can be done through various methods such as using a humidifier, placing water on a radiator, or by simply adding water to the air via evaporation from natural sources like plants and ponds. The purpose of humidifying is often to make the environment more comfortable, especially during dry weather when the air may feel cooler than it actually is due to low relative humidity levels.