The poem imagines a huge, mindless beast racing forward and devouring light. The speaker is blunt about it: the beast can't be taught, trained, or stopped, and it certainly can't be made to love what it eats. The repeated line "the beast will run on" hammers that point home like a verdict.
Yet the middle stanza offers something stranger than despair. If you throw yourself in its path as a "light-seed," you get eaten, but something survives the process. Some of the dark "will part in its wake," and the speaker says you have given darkness "a horizon." Sacrifice doesn't defeat the beast, but it creates a edge, a place where dark ends and something else can begin.
The mood is bleak but oddly steady. The speaker doesn't sound panicked; they sound resigned and clear-eyed. The beast could be time, history, or any relentless force, though the poem leaves that open. What it suggests is that small acts of giving can leave a mark even when nothing can change the larger course.