I didn't know what one of these was until a few days ago, although our pet-savvy folks probably did. It instantly gives another level to a small-animal cage, making it functionally larger. Much cheaper than buying more cage for a pet. Or, you could make one.
At each corner of this one is 3/8-inch wide elastic 6 inches long, attached at the end to a 1.5-inch-long snap hook of the type on dog leads, or I've seen others with simpler snap hardware, like lanyard snaps. You thread each through and around as far as you need, to attach to some outer part of the cage, so the hammock hangs where you want it. Says it is "for ferrets, rabbits, cats, small dogs, rats, guinea pigs and chinchillas".
You can see these small animals prefer it, even though it is their 3rd, top level, right at the roof of the cage. Their solid 2nd level is available below them to the left, but they prefer the hammock, so the 2nd holds their water. With three levels, the litter box (right) can be far from the sleeping place.