The poem sets two images side by side: the dead and the living. In the first stanza, the old people lie in "fine hard boxes," and the speaker notes how harmlessly they do it. The word "harmlessly" has a dry, slightly uneasy edge. Death makes people quiet and safe, but it also makes them useless to themselves.
Then comes the turn: "yet I believe in wine and cake." The "yet" pushes back against the coffins. Against all that stillness, the speaker chooses pleasure, the plain physical delights of eating and drinking. It sounds almost like a declaration of faith, with wine and cake in place of religion.
The last two lines add a funny, odd twist. At a banquet, the speaker feels jealous of their own stomach, which seems to enjoy the food more fully than the mind does. The mood is wry and light, but a shadow of mortality sits behind it. The speaker wants to taste life while there is still time, and wishes to do it more wholeheartedly.