Plain English Meaning
Pulling the wheels up into the airplane after takeoff so they're tucked away during flight.
Full Definition
The act of mechanically raising the landing gear from its extended (down and locked) position into the stowed (up and locked) position within the airframe after takeoff, typically commanded by the pilot moving the landing gear selector to the UP position once a positive rate of climb is established and there is no usable runway remaining.
Context Anchor
You encounter this term when operating an airplane with retractable landing gear, especially during the after-takeoff flow and when reading procedures for landing gear operation.