As the air passes through the filter, it evaporates some of the water there. The higher the relative humidity, the harder it is to evaporate water from the filter, which is why a humidifier is self-regulating -- as humidity increases, the humidifier's water-vapor output naturally decreases.
Sometimes an evaporative humidifier will be hooked up to the heating and cooling system of a house or building. These systems work in a similar way: A metal mesh or screen is located in the duct coming from the furnace and/or air conditioner; water from the building's pipes flows down the screen; as air coming from the duct blows across the screen, it picks up moisture.
Next we'll take a look at a few other types of humidifiers.