Basically, they come in a small bottle that the Domme puts up or near your nostril & you inhale the fumes. Dr. Pleasure is right about the science but to put it in simpler terms, it relaxes your asshole which makes it easier to take larger strapons/cocks.

The other thing that is good about Poppers is that it's sort of like getting laughing gas at the dentist in that the effects dissipate relatively quickly so driving after a session is safe (unlike other types of forced intoxication).