What does pitot heat do?

From FlightSim

Pitot heat is very necessary. The pitot tube is used to measure your airspeed.

If the opening in the tube is blocked it will not work, and your airspeed will drop to zero. This usually occurs in icing conditions. Pitot heat electrically heats the pitot tube and prevents this. To see it in action, set up a cloud layer with icing and fly through. In no time your pitot tube will ice up and the airspeed will drop to zero. Turn on the pitot heat and wait for the ice to melt.

Michael D. Klatt