This page of the Assassin's Creed Valhalla game guide includes a detailed walkthrough description of the Old Man on the Edge side quest.
Reward for completing the mission: none.
In the place marked in the image above you will meet a man named Harek. He seems to be a freak who first pulled up a lot of crates with different objects just to drop them off. As he himself says, they are to be sent to another world.
Go up to the chests and pick them up one by one and then throw them into the chasm. Talk to Hark again when the job is done.
Interestingly enough, Harek will say that the throwing away of the stock was only a preview of his arrival to Asgard. A man takes a run-up and then jumps off a cliff and commits suicide - you can't do anything to save him.