Plain English Meaning
When ATC says 'squawk,' they're telling you to enter a number into your transponder so they can see you on their radar screen with that code attached.
Full Definition
An instruction from air traffic control directing the pilot to set a specific four-digit code on the aircraft's transponder, which then transmits that code to ATC radar so the flight can be identified and tracked.
Context Anchor
Heard in ATC instructions during IFR clearances, departures, radar contact, and when a controller needs you to change your transponder code.
Derivation
The term comes from 'Parrot,' the World War II codename for the early IFF (Identification Friend or Foe) transponder . Operators talked about making the 'parrot squawk,' meaning getting it to its identifying signal. The nickname stuck and is still today.