This poem is spoken by the grass, and it's all about the job that the grass does—and this job is growing. But the grass is a symbol of so much more. It represents new life after the death on the battlefield. This isn't necessarily a good thing, though, because the grass has the ability to erase the signs of death and destruction with its awesome green lusciousness. And for Sandburg, the erasure of history is most definitely a problem.
Lines 1-6: This grass has a one-track mind; after a battle, it wants to get to work and "cover all." And by "all," it means all of the signs of death and destruction from bloody battles—including dead bodies.
Lines 7-9: We see in these lines that the grass is successful in its work; train passengers travelling by battlefields have no idea that they're even seeing battlefields. Nice work, grass.
Lines 10-11: The grass repeats its favorite line "let me work." And now that we understand the grass's work, it's up to us, the non-grassy humans, to remember war and battlefields in ways that the grass, and nature, can't.