Something that represents human
babies, the
organs used to
conceive and
nurture them (
penises,
vaginas and
breasts, most commonly) or growth of
crops and
livestock. The
Venus of Willendorf Paleolithic sculpture is one of the most well-known fertility symbols; it's a "squat little figurine of a woman with what we would now call 'child-bearing hips'" found with red
ochre (representing the blood of
menstruation?) on it, which probably represented the kind of woman (or goddess) who had given birth.
Other fertility symbols are more subtle; a cowry shell as a "womb gate," a maypole as a phallic symbol, a lotus flower because Brahma sat on one while creating the world, frogs because they seemed to appear out of nowhere after rains, and many more throughout the world. A complicated mythology often develops to explain why a particular thing is used in a ceremony or offered to a god (see James Frazier's The Golden Bough for many examples), but many of these items started out because of their association with fertility.
Sources:
http://arar.essortment.com/fertilitysymbol_ricd.htm
http://www.pubergen.com/symbol.htm