What does it mean that Satan would bruise the heel of the seed of the woman?
It means Satan would cause the suffering that Christ, the seed of the woman, had to endure for our salvation. Christ would crush Satan's head through His death and resurrection, defeating him, but Satan would still bruise His heel by inflicting suffering.
Les explains that the seed of the woman is the Christ, the Son of God, the coming Redeemer promised as soon as man sinned. The remedy for man's sin was that the seed of the woman would be crushed, put to death at the cross. Through His resurrection power, He would bruise Satan's head and put Satan in a place of defeat. But Satan would come back and bruise His heel—that is, cause the suffering Christ had to go through for our salvation. As Les illustrates, a snake is really killed only by crushing its head, yet the serpent got his licks in by causing Christ to suffer so inexorably.
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.