This is a little article I found on it. It gives a pretty good description of it.
Penis Ring or Cock Ring
A cock ring is most often used to make an erect penis harder and bigger, to keep it that way for a longer amount of time, and to delay and heighten orgasm. Cock rings work by constricting blood flow, keeping blood in the shaft of the penis. When a wearer finally does ejaculate, the sensation is more intense, since it took a longer time to come, and because the penis is filled with additional blood.
Men use cock rings with partners and/or by themselves. Some are actual rings as their name suggests, made of metal or rubber. Since these cock rings are not adjustable, the right size needs to be chosen for a proper fit. For example, one that fits too tightly could be hard to remove and could cut off circulation in the penis entirely. Metal rings can cause a rash or other skin reactions in some men, so this is something to consider when shopping around. Adjustable cock rings made of rawhide or leather straps are easier for beginners to use since the fit can be changed, and taking them off is more manageable. Avoid using cock ring substitutes; rubber bands, binder clips, and vices, for example, cause too much constriction.
Cock rings fit over the shaft and the testicles/scrotum when a man is not erect. Ball by ball, the testicles go through, then the penis shaft dives in head first through the remaining ring space. The ideal cock ring stays in place when the penis is soft, yet it can fairly easily slip off or unsnap when the penis is hard. Lube can be added to make putting rings on and taking them off a little easier.