On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough of the Wincestre side quest: Romeo and Aetheflaed.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet Esobert on top of the wooden scaffolding. The boy climbed so high because he wants to get a flower for his chosen one.
It looks like Esobert lacked a bit of courage, so you'll have to help him out. The task is to slide down the rope at the right moment and land on a tree branch.
Collect the flower, jump down and talk to Esobert. After a short cut-scene the mission will end.