Some say this is an allegory about Jesus. Some say this is an example of God stating Jesus as God's only son.
This is, of course, one of the most puzzling stories of the entire Hebrew Bible. Why would a loving God ever ask such a sacrifice—particularly of someone with whom he established the covenant that is repeated at the end of this passage? I've heard it interpreted as a warning against accepting without question everything that may seem like spiritual guidance but may really be a projection of your innermost fears.
I choose to think of it as an affirmation that even the most precious aspects of our human experience—as Isaac certainly was to Abraham—cannot take precedence over our spiritual commitment and spiritual purpose. If we walk steadfastly through any fear or sense of loss for the sake of our spiritual journey, we will find a loving response and a renewed sense of the covenant made when we, as spiritual beings, agreed to enter into this earthly experience.
Many people see—quite correctly, I think—in this story of a sacrifice asked and then released a foreshadowing of the sacrifice of Jesus, which was carried through to the end. I don't see at all that the story makes a statement about a unique relationship between God and Jesus. For one thing, Isaac was not Abraham's only son.