In Abraham’s day covenants were more powerful and binding than contracts. In a contract both parties agreed to do their part. If one party failed to do his part the contract was void. In a covenant, both parties agreed to do their part even if the other party failed to do their part.
To ratify a covenant the parties divided sacrificial animals in half with an open pathway of animal blood between them. Then both parties passed between the animals together to seal their agreement. The parties were indicating they were willing to fulfill their covenant promise even if it cost them their lives.
When it was time to ratify the covenant God made with Abraham, God caused Abraham to fall into a deep sleep. God passed through the sacrificial animals and the covenant blood alone. By sealing the covenant alone, God was promising to do all the work Himself. God knew Abraham was incapable of keeping the covenant. Only God could keep the promises God made to Abraham (See Genesis 15).
As children of God we are parties to the covenant God made with Abraham that day. All we need do is trust Him and receive what He promises. He will keep His promises and do everything alone.