The Widow's Son is Freemason code for Hiram Abiff, chief architect (i.e. "master mason") of Solomon's Temple, who was murdered by three jealous builders. However, the term "The Widow's Son" equally applies to Jesus Christ, (possibly) Merlin, and Perceval, interestingly enough, who are all integral parts of the grail romances, particularly that of Robert de Boron's Le Roman du Graal.
The Widow's Son is he who is born without a father to raise him. He is often raised away from society, which gives him special powers unknown to most people. He is special, chosen by God to fulfil the divine plan which is still being sought out by Kabbalists.