Fireflies rise from the grass like earthbound imitators of the stars. Frost sets up a gentle comparison: the sky fills with “real stars,” while the garden fills with small lights that can only copy them. The speaker isn’t mocking the fireflies. He admits they “never equal stars in size” and were “never really stars at heart,” yet they can manage “a very star-like start.” The verb “start” matters—it suggests a flash, a sudden shine, even a small leap toward glory.
The mood is tender and lightly amused, with a faint sadness in the final line. Fireflies can sparkle for a moment but “can’t sustain the part.” The poem quietly observes that some things are beautiful precisely because they are brief and imperfect, like little actors trying on starlight and losing it almost at once.