A cock ring is a ring, a few inches in diameter, encircling the base of the penis.
You put it on when the penis is flaccid, pushing it back as far as possible, and hold it in place till your erection secures is. When you have a hard on the ring is snug, but not too tight.
Some cock rings not only encircle the penis, but also the balls.
With a tight ring the penis gets more engorged than normally, blood flowing in but not out as quickly.
The effect on orgasm differs. Some find it more difficult to climax, some say it hurts a little while others report that it intensifies the sensation of orgasm.

WARNING!
Why do guys wear a cock ring?