Genesis 21:29 Meaning

27And Abraham took sheep and oxen, and gave them unto Abimelech; and both of them made a covenant.

28And Abraham set seven ewe lambs of the flock by themselves.

29And Abimelech said unto Abraham, What mean these seven ewe lambs which thou hast set by themselves?

30And he said, For these seven ewe lambs shalt thou take of my hand, that they may be a witness unto me, that I have digged this well.

31Wherefore he called that place Beer-sheba; because there they sware both of them.