Ultima is the name of a series of computer games, originally written and designed by Richard Garriott. Over the past few decades, the game has seen a number of iterations, starting with Ultima I in 1981 and ending with Ultima IX in 1999, but also with a number of 'spinoffs' like Ultima Underworld and the still running online MMORPG Ultima Online.
The games revolve around people from Earth (first Lord British, Iolo, later the Avatar) who are summoned to the alternate world of Sosaria or Britannia. There they are called upon to save the world from harm. As visitors from another realm, they live far longer lives as time progresses differently for them there than it would on Earth. Lord British, the first visitor to the realm of Sosaria, ruled the land for hundreds of years, with the Avatar travelling between the realms and even to some others during that time.
In Ultima: Resurrection, the Avatar is gone. He has ascended to another plane. The citizens of Britannia are left to find virtue for themselves now. That is where you come in.