The Amalekites came and attacked the Israelites at Rephidim.
Moses said to Joshua, "Pick out some men to go and fight the Amalekites tomorrow. I will stand on top of the hill holding the stick that God told me to carry."
Joshua did as Moses commanded him and went out to fight the Amalekites, while Moses, Aaron, and Hur went up to the top of the hill.
As long as Moses held up his arms, the Israelites won, but when he put his arms down, the Amalekites started winning.
When Moses' arms grew tired, Aaron and Hur brought a stone for him to sit on, while they stood beside him and held up his arms, holding them steady until the sun went down.
In this way Joshua totally defeated the Amalekites.