Plain English Meaning
Turn on the part of your transponder that sends your altitude to the controller's radar.
Full Definition
An ATC instruction directing the pilot to activate the transponder's altitude reporting feature (Mode C), so that the aircraft's pressure altitude is transmitted along with the assigned transponder code.
Context Anchor
Heard in radio instructions from air traffic control when the controller wants your transponder to report altitude on radar.
Derivation
Squawk' comes from the World War II identification system code-named 'Parrot,' so transmitting an identifying signal became known as 'squawking.' The word stuck and is now for transponder instructions.