The altar is spoken of as if it were a witness that could testify to the rights of the three tribes. Alternate translation: "to remind us and you, and our descendants and your descendants" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The phrase "before him" here means in Yahweh's temple. They would not offer sacrifices on the altar that they had just built. Alternate translation: "that we will perform the service of Yahweh in his temple"