an elastic ring which fits around the base of the penis. It constricts the veins which drain blood from the penis, thereby strengthening an erection. Bought cheaply in automotive parts stores as "O-rings", they are black in colour. Also available in sex shops which sell vibrators, condoms, etc.Also increases the sensation during ejaculation as it constricts the urethra.
I had been having trouble maintaining an erection with her, as I was very nervous. We had some wine, which helped, but then she cut the end out of a condom and rolled it backwards onto me, forming a cock ring at the base of my dick. After that, I had no problems.