Plain English Meaning
The small forward-facing tube that feeds the airspeed indicator can ice over in cold, wet conditions and stop working. A heated pitot tube has a built-in heater the pilot switches on to keep it clear so the airspeed reading stays correct.
Full Definition
Pitot tubes fitted with an internal electrical heating element that prevents ice from forming over or inside the tube's ram-air opening, preserving accurate airspeed indication when flying in visible moisture at or below freezing temperatures.
Context Anchor
You encounter heated pitot tubes during preflight inspection, checks, and or discussions, especially before in cold visible moisture.