1) (noun) Sound supposedly made by mating arctic foxes.
2) (acronym) (or more backronym) YIFF or Y. I. F. F. - "Young Incredibly Fuckable Furry"
3) (proper name) YIFF sound server. Program for Linux to help handle audio.
4) (noun) More or less (usually more) sexual act. Commonly used by furries
, usually denotes standard intercourse but may denote any kind of sexual behaviour between any type and number of parties (imaginary anthro creatures, humans, animals, even inanimate objects like plushies). Antonym: Yerf.
5) (verb) Like in meaning 4, "to yiff".
6) Furry (or otherwise) artwork depicting yiff as in meaning 4.
7) Expression of joy (by furries)
8) Non-invasive question about sexual feelings/will to mate.
Note in all sexual meanings this has a very specific emotional background: It's neither considered vulgar (like "fuck" or technical (like "have intercourse") or emotion-less (like "make love"). It's playful, considering the act like something fun, nice, pleasant and not very important - without any special obligations attached to it. "Let's yiff" has about the same emotional background as "Let's play".
See also yiffy