Sir William has not yet returned from his crusade and obviously he hasn't got anyone capable of running his castle properly. His people are running short on food and it is up to you to keep them well fed.
This mission isn't difficult at all. Build a carter's post at your home estate Harbury and, if you haven't yet, build another one at Slaughterford. Task them to deliver 50 apples to Sir Williams estate. Which is the one over the bridge to the left (!) not the bridge over which the outlaws keep attacking. You might have to wait a few moments for the shipment from Harbury to start, because your granary has been emptied at the start of the mission. If you have more meat than apples at either Slaughterford or Harbury, you can send that instead. As long as you have two carts delivering 50 units continuously, you can't lose this mission.
If you have more than 100 units on honour, you should purchase the northern neighbour Boorswell. You'll get it for free in the next mission, but since it has been repeatedly under attack if you didn't follow the alternative approach, you might run into a situation that makes it impossible to win the next mission.