A "rim job" is the slang term used to describe analingus, sort of like how "blow job" is the slang used to describe fellatio.
But, we already know what a blow job is. What exactly is analingus?
Put bluntly, a rim job is when one partner orally stimulates the other person's anus. So, it's licking the butt hole.
Why Would Someone Do That?
Well, to some, a rim job can be quite pleasurable. The anus has a lot of sensitive nerve-endings that respond positively to physical stimulation.
Just as you might stimulate other sensitive areas on your partners body, a rim job is simply one more option.
Is it safe?
A rim job is a risky form of oral sex, and protective measures should be taken if you plan on experimenting with analingus. Fecal matter can easily transmit pathogens to the giver of analingus, causing countless diseases or infections.
If you're going to experiment with rimming, use a dental dam or other recommended form of barrier to avoid direct contact with the anus.
Is it Normal?
Though the majority of people don't regularly practice analingus, it is a completely valid sexual activity. You should never feel ashamed, or shame your partner for desires that don't seem main stream.
As with all sexual activity, it is key to maintain a healthy, open line of communication with your partner, and only perform activities that you both feel comfortable with.