Unrated stuff means there are scenes that were never shown to the people that rated the movie(as in like a pg-13 rating) or the movie might not have even shown to anybody to be pre-screened.
movies arent rated in theaters, they are rated before hand by the motion picture association of america before the movie is ever released to anyone. the unrated ones are movies that were never viewed by the MPAA, because they have very violent or shocking content and probably wouldnt pass to be shown in theaters, so they are released directly to DVD
Actually no film HAS to be rated by the MPAA. It is the smart thing to do because many theater chains will not take unrated or NC-17 rated films to show.
So, studios or filmmakers submit to the MPAA and are given a rating. If it's too high a rating they may take, say...an R and cut it down to achieve a PG-13. Direct to video films do not have that same kind of scrutiny. There is nothing forcing their hand. It is also why you'll see films which were released as a high R come out on DVD as Unrated. They can push the envelope more that way.
Take a movie like "Walk Hard." The theatrical version was raunchy but to a limit. The unrated DVD had much more nudity, including the big no-no of full frontal male nudity. Unrated can really go to town. But the MPAA has it's limits.