Official Aviation Definition
A pitot tube is a forward-facing open probe mounted on the exterior of an aircraft that captures dynamic ram air pressure generated by the motion of the aircraft through the air. Connected to the airspeed indicator, it works alongside static pressure ports to measure indicated airspeed.
In Simple Words
A small forward-pointing tube on the nose or wing of an airplane that measures how hard the wind is rushing into it to calculate airspeed.
Real-World Flight Example
Pre-flight walkarounds always include verifying that pitot tube covers are removed and drain holes are clear of insect debris.