On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough of the Snotinghamscire side quest: An Efficient Cremation.
Reward for completing the mission: 1200 XP.
In the place, as shown in the image above, you will meet a Viking. His loved ones died an unjust death, and he wants to burn them, so they get a decent burial, but the fire is too weak, and he doesn't know how to fuel it. Optionally, you can read the letter you find in the chat room.
Behind the large red jugs, you will find a smaller jug of oil. Take it and throw it at the cart to fuel the fire. Talk to the Viking, and he will admit that he killed his family himself and will tell you why he did so. Moreover, the Viking will want to join his loved ones - fulfill his request and the quest will end.