Prologues are all about "laying pipe:" explaining the back story and how the novel's world works.
However, if you read agents' blogs, you can see that agents are very anti-prologue these days, so I always advise writers to get right into the story and reveal back story and rules of the world slowly, over time.
Of course this is easier said than done. Science fiction and fantasy in particular require world-building because the settings and their rules tend to be unfamiliar to readers. One way to ease into back story, though, is to reveal the unfamiliar world as the protagonist discovers it. This technique works when the world is new to the character, such as in Alice in Wonderland and Through the Looking-Glass, where Alice discovers the fantasy world after she falls through the rabbit hole/goes through the mirror. As she explores, she describes everything she sees and everything that happens, so we learn at the same time she does.
When the characters already live in the fantasy world, the key is to get readers to care about them as quickly as possible (see screenwriting guru Blake Snyder's Save the Cat! books, in which he advises that your protagonist do something kind or sympathetic ASAP so we see his goodness or vulnerability and bond with him/root for him; this technique gets us hooked). Then you reveal the world and the back story. Once we care, we'll be more forgiving of a few digressions--as long as they're relevant to what's going on at the time and aren't too info-dumpy.
A useful trick to get readers to stick with you while you reveal back story and/or the rules of the world is to get your narrator or the viewpoint character to muse about what's going on. You can sneak explanation into the musing. For example, "Of course Brunhilde wasn't allowed in the castle. The only commoners who had ever set foot inside were members of the Ningning family, and for good reason. During the wars between the barons and King Whoop-de-doo, the Ningning family was responsible for hiding and protecting the king's son and heir, Prince OMG, and his descendants, including the present king, Bling II, weren't about to forget their kindness. Only one problem: Brunhilde wasn't a Ningning."