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/jit/figs-metaphor]])
The phrase "before him" refers to the place where Yahweh had said they should offer sacrifices to him. 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 tabernacle"