Plain English Meaning
The part of a controller's radar screen where written information about each aircraft (like its call sign, altitude, and speed) is shown next to the dot representing the aircraft.
Full Definition
On an ARTS III or DBRITE radar display, the readout area is the portion of the screen dedicated to displaying alphanumeric information about each tracked aircraft, such as its identification, altitude, and groundspeed, presented in a data block next to the aircraft's radar target.
Context Anchor
Seen in the Instrument Flying Handbook figure showing an ARTS III / DBRITE radar display used by air traffic controllers.
Derivation
Readout' comes from the idea of a machine 'reading ' information to a human in a form. In radar, it refers to the text the generates and shows on screen alongside the radar return.