The speaker is thinking about the body as a battleground between living and dying. Their organs and limbs are in pain, "moaning" as they are kept going, while the bones are already looking ahead to the grave, which the poem calls a "barrow." It's a striking split: the body is divided against itself, with the working parts clinging on and the skeleton ready to rest.
The "game" in the title and middle lines suggests a contest with its own rules and competing interests, though the poem never says who the players are. Judging by the imagery, one side is the will to keep living and the other is death, or perhaps simply the body's slow wearing down. The ending reverses the usual idea of defeat. Death wins, and the speaker finds "comfort" in that.
The mood is weary and quietly resigned, with a dry, almost dark humor in treating mortality as a game. It feels less like fear than like tired acceptance.