This is different than a rimjob. A rimjob would, however, be a precursor to anilingus. It is the act of plunging one's tongue into the anal cavity of your sexual partner and then swirling and thrusting aforementioned tongue in an effort to make
said partner
happy.
It is well advised to wash the anus that is to be pleasured thoroughly (and perhaps
even douche) before attempting this.