What do you not get about it? The teacher tells him about cellar door, which he goes to. Frank does everything because it sets things into motion; Donnie must live so that he could go through everything and eventually send the engine through, killing himself, and thus creating a paradox. Frank is a Manipulated Dead, and Donnie kills the Frank in the Tangent Universe so that Frank can come back as a Manipulated Dead and guide him. If Donnie does not do this, the Tangent Universe will condense into itself into a black hole and most likely wipe out all existence.

You have to read the Philosophy of Time Travel pages on the official site, which is nearly impossible to navigate without assistance.