A yoni is like a phallus, only female.

Anything vaguely oblong appears phallic to our post-Freudian eyes. Anything with a hole may legitimately be called yonic. Donuts, for example, are yonic. In fact, any significant indentation qualifies. Thus, bellybuttons are yonic too.

Though the phallic and yonic tendencies seem to be opposites, it may be noted that their union often brings about pleasing results.

Notes for further study:

Actually the more exact male analog for the yoni is the lingam, but I didn't think you'd recognize that word. Yoni and lingam are Sanskrit words that have made their way into English usage. They refer to representations of the female and male sex organs used in the worship of Shiva and Shakti.