Rimming is another name for anilingus which is a form of anal sex and oral sex. This involves the use of mouth and tongue to give pleasure to your partner's anus or the surrounding areas. This includes the act of kissing, licking, sucking among other ways to stimulates the pleasure points. It is called rimming because it is done though the rim of the anus. Rimming is a sexual activity that is enjoyed by people irrespective of their gender identity and sexual orientation. Rimming can transmit STIs and use of dental dams is recommended. Use of Lube is also encouraged as the anus is not self lubricant.
Sources:
https://www.healthline.com/health/healthy-sex/what-is-rimming#definition
https://www.verywellhealth.com/what-is-rimming-5085084
https://www.menshealth.com/sex-women/a23652027/what-is-rimming/