Several meanings, determined by how it is used, but basically in relations to sex, and animals, either in actuality or in depiction or caricature.
In the gay parlances, a zoo number can simply refer to wild sex, where the boundaries are unlimited. It does not mean having sex with animals, in this particular manner.
It can also be in reference to one's sexual partner, in that they were wild, uncontrollable in their sexual prowess.
In the straight lingo, it can refer to one's date as being ugly, or being unattractive, again not in reference to actual sex with an animal.
It can, depending on how it is being used, refer to a person who seeks and has sex with animals.