Genesis 3:15

And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.

1. The LORD God apparently did something to cause a feeling of ill-will or hostility between Eve and the serpent, and between Eve's posterity and the posterity of the serpent.
2. The phrase, "her seed", seems to refer to a specific, male individual, since "it" would bruise the serpent's head, and the serpent would bruise "his" heel.
3. I assume this bruising of head and heel either refers to some incident not documented in the bible where a snake gets its head stomped on for biting someone's heel, or that it is symbolic of something else (I would guess it's the latter).