Multiple interpretations can come out of an analysis of the Grasshopper in Keats's poem "On the Grasshopper and Cricket," but this question can be more accurately answered when considering the Cricket's symbolism as well.
The sonnet emphasizes the immortal beauty of nature: "The poetry of earth is never dead," and "The poetry of earth is ceasing never." Each of these lines precedes the introduction of a new season (summer and winter) and central character (Grasshopper and Cricket). If one approaches the sonnet as a microcosm of a loose narrative, the Grasshopper's symbolism can be discerned.
The Grasshopper and the Cricket operate as both literary foils and mirrors. A literary foil is a character that is meant to contrast with another character's characteristics. A mirror is meant to complement another character's characteristics. The Grasshopper does this by representing the joy and beauty of summer from a third-person perspective ("When all the birds are faint with the hot sun, / And hide in cooling trees, a voice will run"). This is in heavy contrast to the Cricket's current situation during the winter ("On a lone winter evening, when the frost / Has wrought a silence, from the stove there shrills / The Cricket's song").
The literary mirror is represented through the Grasshopper and Cricket's similar anatomy (they are both insects) and shared tendency to "sing."
The summer is generally considered a more beautiful season than the winter, but the Cricket's wintry song reminds us of the Grasshopper's summer-driven song and that the poetry of nature persists through even the most different of seasons. The Grasshopper symbolizes one facet of a connected whole.