Okay, so on the one hand, the title of The Final Frontier is simply effective branding. In the opening narration of the original Star Trek series, Captain Kirk refers to space as "the final frontier." Why not turn that sweet phrase into a title, right?

On the other hand, the idea of a final frontier does resonate nicely with the film's plot. The whole movie is about Sybok's quest for God, which, in Star Trek's conception of things, would definitely be the final frontier of human knowledge. Future humans know everything there is to know about science—but they know a lot less about metaphysical matters.

Of course, that quest is ultimately revealed to be a farce, but the title still enriches our understanding of the characters' motivations—and maybe our own, as well.