A Cage Nut is used on a rack that has square mounting holes. You would use a cage nut when you need to mount a product in your rack that requires a threaded hole. A cage nut is nothing more than a square nut that has threaded holes wrapped in a piece of spring steel designed like a cage. This spring steel has two flanges that when pressed together allow the cage nut to be inserted into the square hole and simply clip in. When this done you now have a square hole that is threaded. The nut is typically loose in the cage to allow for adjustments when installing the screw.
There are 3 common sizes for cage nuts. These sizes are 10-32, 12-24 and M6. The size you would need would depend on what is required by the product you are mounting. I always recommend purchasing the corresponding screw to go with the cage nut. This always make the installation easier if you happen to choose the wrong size cage nut.