A Hand was a member of the One Sith around 130 ABY. They were Darth Krayt's most capable servants, trained and existing to carry out his will. They followed only his orders, and reported only to him.
By 137 ABY, Krayt's only Hand was Darth Nihl, having been given the post when his predecessor died unexpectedly during the Massacre at Ossus. That same year, he made Darth Talon, a Twi'lek Sith Lord, into his second Hand, a fact that displeased Nihl.
After both Talon and Nihl were grievously injured by Cade Skywalker, Emperor Krayt named Darth Stryfe as his new Hand to replace the demoted Lord Nihl.