Genesis 21:28 Meaning

26And Abimelech said, I wot not who hath done this thing: neither didst thou tell me, neither yet heard I of it, but to day.

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.