Plain English Meaning
A small built-in heater that keeps the airspeed sensor on the outside of the aircraft from icing up. The pilot turns it on with a switch so the airspeed reading stays reliable in cold or wet conditions.
Full Definition
An electrical heating element built into the pitot tube that prevents ice from forming in or over the tube's opening during flight in visible moisture or freezing conditions. When activated, it warms the pitot tube so that the ram air inlet stays clear and continues to deliver accurate pressure information to the airspeed indicator.