Clueless is an in-setting slang term in the Dungeons & Dragons setting of Planescape which is used by the denizens of Sigil to refer to any migrant to their city, or any of the Planes, by denizens of the Prime Material Plane. Technically, it's the ruder of two such terms; the term "Prime" means "anyone from the Prime Material", whilst "Clueless" means "a Prime who is spectacularly ignorant as to how things work on the Planes, often to the annoyance of those around them and/or their own endangerment". Think the difference between a country bumpkin visiting the big city for the first time, and a country bumpkin visiting the big city whilst refusing to believe that they're out of their depth or that things work differently.
Of course, since Sigil's natives tend to be a bunch of elitist snobs, you hear the term "Clueless" a lot more often, so it sticks. Whilst technically the Clueless aren't one of the factions, they're common enough to be considered almost an honorary faction - though it's usually referred to in polite company as "The Outsiders".
The fledgling sect known as the Planarists is characterized by its unusually intense xenophobia in regards to Primes; its defining belief is that all beings from the Prime Material should be banished to their own plane and kept from ever entering Sigil.
Naturally, the fandom responded with the tongue-in-cheek concept of the Prime Regals, a Prime-exclusive Sect who argue that that Prime Material is the true core of the multiverse and generally want to stick up for themselves in the face of planar elitism.