Frottage refers to the act of rubbing against someone else for sexual arousal, pleasure, and/or satisfaction. It comes from the French word frottage meaning rubbing or friction.
The word was originally used to describe a non-consensual fetish where an individual would get sexual arousal and/or satisfaction by rubbing up against other people in public places, usually rubbing their genitals, and usually fully clothed. Today the word frotteurism is used to describe this non-consensual fetish.
The word frottage is now used to describe a consensual sexual act, also sometimes called dry humping, in which two people who are usually fully or mostly clothed, rub their bodies together. Frottage is distinguished from tribadism by the fact that the people engaging in it are usually fully or mostly clothed.
Frottage is thought to be a common and pleasurable sexual activity when done consensually. And when fully clothed it has the benefit of being a sexual activity that comes with almost no risk of any harm (except possibly some irritation from fabric, or problems with zippers).