Usually refers to cyber sex between two or more members of the furry fandom. Originally refers to the sound that foxes make when mating. Members of the furry fandom may use the term to refer to actual sexual contact, usually in a lighthearted manner.
It is also a term used at furcons to inquire about sexual intentions between fursuiters.
The anthro fox smiled at the mass of horny furres, knowing that a good yiffing session would soon follow.