Original: Some sources say that Edward had verbally promised the throne to his cousin, William, the Duke of Normandy, but decided on his deathbed to give it to Harold. While Edward the Confessor had an English great-nephew who might have qualified as his successor, he was deemed too young.
Simplified: The ruler of the Normans, William of Normandy, claimed he was the heir to the English throne as his cousin, Edward the Confessor, had promised him the throne but changed his mind on his deathbed.