The Director doesn't even tell us what the whole movie means...I think you have to take from it what you will. To me the whole movie is about how Donnie is being shown what is going to happen in his future and kind of being asked if he wants his future to continue...He gets to meet Gretchen who he falls in love with. This girl who totally understands him and loves him back but he can see that she will die and it will be the worst pain he's ever felt in his life...instead he choses to die being crushed by the airplane engine and to never know her than to suffer that heartbreak. I also think it's sort of about Donnie's struggle with life, how people in his town are really messed up like freaky child molesters who try to make him think a certain way . He just doesn't fit in. But When frank says that the world will end...he doesn't mean everyone's world he just means Donnie's.

how long an answer do you want? - In fact a lot of article's and books have been read about this same subject. If you watch the DVD together with the comments of the director you get an idea/although the ideas of the director aren't necessarily important.
I take something different from it every time I see it, which is something that I think is an important quality in a good film. 1. theory is that there is a rift in the structure of time and space as Donnie should have died from the plane engine from nowhere, when this doesn't happen the world is unravelling and Donnie suspects this, the only way to make it good is by dying in the end - the world (his world) dies at that end.
The elussions to Frank are also interesting as he is the one who is dating D. sister (when she comes in after her date you hear the hunk of his horn) and the one who kills Gretchen and who Donnie ends up killing....(I seriously could go on and on, so just watch it over and over again, read articles about it, etc)