What does Genesis 22:4 mean?

After these things, God tested Abraham, and said to him, “Abraham!” He said, “Here I am.”

He said, “Now take your son, your only son, whom you love, even Isaac, and go into the land of Moriah. Offer him there as a burnt offering on one of the mountains which I will tell you of.”

Abraham rose early in the morning, and saddled his donkey, and took two of his young men with him, and Isaac his son. He split the wood for the burnt offering, and rose up, and went to the place of which God had told him.

Abraham said to his young men, “Stay here with the donkey. The boy and I will go yonder. We will worship, and come back to you.”

Abraham took the wood of the burnt offering and laid it on Isaac his son. He took in his hand the fire and the knife. They both went together.

Isaac spoke to Abraham his father, and said, “My father?” He said, “Here I am, my son.” He said, “Here is the fire and the wood, but where is the lamb for a burnt offering?”