Captive Nuts are also known as a Cage Nuts or Clips Nuts and they are multiple-threaded fasteners used usually on many thin materials. They are mostly square nuts in a spring steel cage which wraps around the nut. The cage is built of 2 wings that when compressed allow the cage to be inserted into the mounting hole and when released, secure, lock and hold the nut is desired position.
Benefits of using captive nuts instead of threaded holes:
If the screw is over-tightened, the nut can be simply replaced. Pre-threaded holes become unusable in a similar situation.
Captive nuts are easy and safe to use on thin and soft materials where pre-threatening isn't possible.
The nut is usually slightly loose and allows for minor alignments and adjustment if necessary. Pre-threaded holes do not allow for any adjustments once installed.
Captive nuts are commonly used for example for mounting rails of equipment racks and general machinery production industry.