This is a great little video by Dr. Linsey Doe on how to eat ass. Ass eating, also known as tossing the salad, playing the rusty trombone, cleaning the kitchen, analingus, culosex, rimming, licking bud and analinction is licking the anus of your partner for sexual pleasure. The anus has a ton of nerve endings and is an incredibly sensitive erogenous zone — meaning it feels really good to have it licked by something warm and wet. Inserting your tongue inside the anus isn’t necessary but I personally recommend it from an entirely hedonistic point of view.
This explains how to do it, the safety issues and concerns and how to use a dental dam.