Anal licking is also called "rimming" or "ass to mouth". The anus is a very sensitive area, and can be highly erotic for both men and women. This is not a painful activity, and the Master or Mistress is expected to clean well before playing. Depending on the dominant, the slave may either be required to do the licking, or receive the dominant's attention.
Anal licking is all about pleasure and domination. Not only is the feeling of a tongue on your ass incredibly erotic, but there is the added thrill of the taboo. The Master could be forcing their slave to perform, simultaneously enjoying both the pleasure of the act, and their ability to force their slave girl to do something they have always thought of as "dirty" or "naughty". Of course, it can be just as much fun the other way around. The Master uses their tongue to tease and excite their innocent and naïve slave, bringing them pleasure in ways they never would have believed or accepted under normal interactions.