A boxed-in radiator is one that inside a covering that hides it from view. Normally a wooden box will be built round the radiator and decorated in keeping with the room.
If a radiator is boxed in then its valve will not be able to able to sense the actual room temperature. So any smart TRV (thermostatic radiator valve), fitted as part of a multi-zone heating system, will not work well.
You may also want to read our FAQ “What happens if all radiators in a room (zone) are boxed in“