On Christmas day 1066, William Duke of Normandy became King William I of England. The English did not want him as their King and he had to be careful at all times in case they attacked him.
As King of England, William owned all the land and could do what he wanted with it. So he decided to keep one quarter of it for himself and to give the rest to those who had helped him become King.
Each person had to promise to be loyal to the King and to work for him in some way because he had given them the land.