What does it mean that Satan would bruise Christ's heel?
It means the suffering Satan brought upon Christ at Calvary. Genesis 3:15 describes a running battle between Satan and the woman's seed, Christ. Christ bruised Satan's head at the cross; Satan bruised Christ's heel by causing His suffering at Calvary.
Les explains that Genesis 3:15 is set within God's covenant with Adam after Adam and Eve ate and plunged the human race under the curse. In the center of that covenant is the promise of a Redeemer. God says He will put enmity between Satan and the woman, and between Satan's seed and her seed. That seed of the woman is Christ, so there would be a running battle between Satan and Christ. The seed of the woman would bruise Satan's head, which Christ did at the cross, but Satan would bruise Christ's heel, which Satan accomplished when he caused the suffering of Christ at Calvary. From this Genesis 3 promise of a Redeemer through the woman, Les builds the Abrahamic covenant: it would be through this man and his offspring that the seed of the woman would come.
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.