When the last Anglo-Saxon King of England died without an heir, things were in a great mess. Who was going to be king of England? There were many people who wanted to be king and many battles were fought over the claim to the throne.
One contestant to the Throne was William, Duke of Normandy. Since he was a cousin to the last King he felt that he had a really good claim to the throne. Normandy is in France. William was a Norman, but he also held a very powerful fief in France.
William, with the permission of the King of France, decided to go for it. He gathered an army of 6000 knights and squires and invaded England. The last contestant to the throne in England brought his army to face William near the town of Hastings. William won the battle and soon had conquered all of England. He was king.
One of his first acts was to take away from the Saxon nobles and church officials all the land and wealth that they had, and give it to the nobles and other vassals that had followed and fought for him. Next, he sent out officials to take a census of England to figure out exactly what there was for him to tax. (This was called the Domesday book. It still exists and tells a lot about life in Medieval England.)