Plain English Meaning
The gyro instruments have been locked in place with a knob or lever, so they are frozen and cannot show the airplane's actual attitude or heading until they are unlocked.
Full Definition
A condition in which the gyroscopic instruments (such as the attitude indicator and heading indicator) have been mechanically locked in place by their caging mechanism, preventing the gyro rotors from responding to aircraft movement. While caged, the instruments cannot provide accurate attitude or heading information.
Context Anchor
Seen in instrument takeoff checks and error discussions, especially with older attitude or heading instruments that have a cage or uncage .