What does it mean for a payload to be "caged"? If an aircraft has a movable payload (e.g. camera) attached to it, two of the status modes are caged and stowed.  

Stowed indicates the payload is retracted and or in a locked position.

What is caged?  I have found reference to "caged" in a couple of resources online, but the do not explain the term.  In some documents about passenger aircraft, it refers to cages in the luggage compartment, but this is completely different.
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.