The quote you have identified is very important to the entire work of John Milton because, in this one phrase, lies Milton's entire belief about free will, a vital concept when we come to understand theology and God's relationship with his creation. Talking about Satan in Book 3 of this excellent epic narrative, God tells Jesus how he made Satan "just and right," then saying that this is defined as being "Sufficient to have stood, but free to fall." Thus God makes us and his creatures with everything we need to metaphorically "stand" in relationship with God and maintain our upright character, but at the same time, he has not made us like unthinking robots. Indeed, he has given us the freedom to choose our own paths, and just like Satan, we are free to "fall" away from following God if that is what we choose to do. The key is though that this is our decision: nothing that is forced upon us by God. The choice is ours.