In just six short lines, the poem sets two kinds of work against each other. "We" arrive at the park with "a barber's cruel hands," which suggests people who tidy and cut the land the way a barber trims hair, neatly but without tenderness. The word "cruel" tells us the speaker doesn't entirely approve of this kind of grooming.
Then the poem turns. The real "contractor" is a mole, a creature that works underground, unseen, and on its own schedule. The title, "Groundsman," fits this twist: the proper caretaker of the ground may not be the humans at all. The last line, "it is out of light," is harder to pin down. It could mean the mole works in darkness, or that something is fading or ending. Judging by the wording, the speaker seems to feel a quiet humility, even a little guilt, about human tidiness. The mood is dry, wry, and slightly uneasy, with the mole's patient, hidden labor making the human efforts feel clumsy.