What does it mean for a payload to be "caged"?
For something like a movable camera in a gimbal, or even a gyroscopic attitude indicator, it means that it is physically locked into place so that it can't move. On the gimbals (such as the camera pod in a predator, or other ISR asset) that can mean the camera is rotated into a stowed position, and then a pin is mechanically moved into place to keep the payload from moving.  This is usually done to prevent damage.

For a gyroscopic attitude indicator, you can generally cage it by pulling out a knob, and rotating it to a specific setting.  Again, this physically locks it into place so that it doesn't move.  This is usually done prior to shutting down as the mechanisms keeping the gyros spinning are powered down.  This is to prevent damage as well as give it a safe position from which to start from when power is re, applied to the aircraft.