That is a 3-way switch, there is another switch somewhere that operates the same light/fixture. The red is the common, and goes on the black terminal of a 3-way, Blk/wht go on the other two, but you may need to switch them(blk/wht) to get things to work correctly.
Before you start disconnecting wires, look closely at the switch screws. One screw is a common. Depending on how old the switch is or who made it, this common screw should be black and the other two screws gold. Get a three way switch and make sure you connect the wire from the common on the old switch to the common on the new switch.
Bills wire color description may or may not match your setup.