What does Genesis 21:27 mean?

Abimelech, king of Gerar, had approached Abraham to make a covenant meant to formalize their relationship. Abraham was living in Abimelech's land, at his invitation, in spite of Abraham's earlier deception (Genesis 20). Abraham has agreed to swear not to deceive Abimelech and to deal kindly with him. At the same time, Abraham has expressed his concern that Abimelech's men have taken his well. A water source, which Abraham had established, had been captured by men under Abimelech's command. Abimelech claims he knows of no such incident, and agrees to return the rights to this well to Abraham.

Now Abraham gives to Abimelech sheep and oxen as a sign of this covenant between them. In the following verses, he will give additional sheep specifically to show a clear resolution to the issue of the disputed well.