A cock ring is a small ring that, when places around the cock, is supposed to increase the time an erection can last by constricting the flow of blood out of the penis. If tight enough, they work well enough that before Viagra and the like, cock rings were one of the most common ways to treat ed. Another use for cock rings can be cock and ball torture.
Cock rings can be made out of many different materials. The cheapest rings are usually made of rubber, silicone or similar stretchy materials. This allows the manufacturer to produce only a few sizes, keeping cost down. However, these variants usually do not work too well, keeping many people from enjoying the full benefit. Less flexible variants work better but you need to measure well. Titanium, granite or other inflexible materials work best but require very good fit. Some store even offer custom fit for these materials. Some vendors also produce adjustable cock rings, sometimes called lassos.