A cock ring is a ring made of rubber, silicone, metal, and a whole other host of materials that are designed to be worn around the penis. Cock rings have many uses, but the most common is to restrict blood flow and allow longer and harder erections. They can be used medically, for the treatment of erectile dysfunction, and for recreation purposes as a sex toy. Many cock rings vibrate, allowing them to be used with a partner to increase the pleasure for both parties during penetrative sex, or have an additional ring to hold the testicles (allowing for more pleasure during orgasm).
Cock rings are not without their dangers, however, as they can cut off blood flow and lead to penile injury and tissue death if used for too long. This is why it is advisable to steer clear of drugs and alcohol when using a cock ring and to make sure one does not fall asleep, as these behaviors all cause the risk of serious damage to the genitals.