A dehumidifier works by blowing air across a cold coil (like air flowing through a window air-conditioner). It also has a hot coil where it dumps the heat. When air flow across the cold coil the moisture in the air condenses on the coil and then drips into a collection pan. The problem is that in a cool location like a damp basement this cold coil can ice up. A "low temp" dehumidifier runs in "reverse" every so often so that the cold coil heats up. This will melt any ice that may have accumulated on the coil.