A cage nut is a square nut that has threaded holes wrapped in a piece of spring steel designed like a cage. You would use a cage nut when you have a rack that has square mounting holes and you need to mount a product that requires a threaded hole.
The spring steel on the cage nut has two flanges that when pressed together, allow the cage nut to be inserted into the square hole and clip in. The cage nut is typically loose in the cage to allow for adjustments when installing the screw.