Three short lines, and one strange comparison holding them together. The poem begins with a mast, a tall pole, used to hang venison, a slaughtered deer left to drain and age. Then it leaps sideways: the meat is "as mightily bled / as the night in a viol." So the night is imagined inside a stringed instrument, and somehow it, too, has been bled.
The title, "Night music," helps make sense of that. A viol is an instrument that holds sound in its hollow body, and the poem treats darkness the same way, as something deep, resonant and a little bloody. The images are physical and slightly grim, but not angry. The speaker seems less upset than fascinated, watching how death, labor and beauty can sit side by side.
The mood is hushed, dark and strangely reverent. The poem doesn't explain its link between hanging meat and music, which is part of its pull. It asks you to feel the connection rather than solve it, and the night comes out heavy, drained and full of sound.