What does it mean he will crush your head and you will strike his heels?

It is quite interesting as God starts off by saying that there will be emnity between the serpant and the woman. the crushing of the head and the striking of the heel is related to the conquest of God over Satan this is mentioned in Romans 16:20. If you have any doubt that the snake is Satan (as it is not explicitly said in the passage) he is referred to as the sepent in Rev 12:9. Satan would come against mankind as a whole but God would crush him completely and finally on the day of judgement when he would be thrown into the great firery pit. In Matthew 1 and Luke 3:23-37 the geneology of Jesus is described and we see that Jesus's lineage goes back through Adam (and as we know Eve was taken from Adams rib, so effectively born from Adam). So when it says "HE will crush your head and you will strike HIS heels" God is referring to Jesus.