Like other poems by Marvell, this one is a meditation both on man's connection with nature and on the gap between the natural world and the transcendent world of his desires.
Just as in "The Garden," the speaker communes with the natural world and sees a deeper significance in it than just what it physically presents to him; the glow-worms are a guide to the mower in the darkness but also function as companions to him in his loneliness. The metaphysical element is present not only in the conceit that the glow-worms are lamps in the dark, but in the poet's analogizing them with another part of the natural world—and an unlikely one on first thought, as is typical of the metaphysical poets—the comet. The speaker bonds with the glow-worm precisely because it's a humbler natural phenomenon than the comet, which is thought to portend great events like war and disaster. But as is typical of Marvell, the ultimate point of the poem seems to be that the poet's love is more important than all of this: Juliana has so "displac'd his mind" that even the glow-worms, as benign and friendly as he views them, can't help him find his way.
If there is a classical feature in the poem it is the idealization of an abstract female figure who emerges to subordinate everything in the verses that have come before. Marvell's poetry, like that of Donne and of Marvell's other predecessors and his contemporaries, abounds with references to nature and science, but unlike doing what Donne is sometimes guilty of, Marvell doesn't use these elements as ends in themselves but lets the basic message of love predominate. His analogies are also much simpler and easier to understand than those of Donne and even of Marvell's closer contemporary Abraham Cowley, who often seem more intent on demonstrating their vast knowledge than in putting across a simple message. For Marvell, it's straightforward enough in, for instance, "The Definition of Love," to conclude that the poet's love is "the conjunction of the mind/And opposition of the stars," using terms from astronomy in such a way that the reader hardly even notices that specialized knowledge is being invoked. So it is in "The Mower to the Glow-Worms." The metaphysical conceits are direct and muted enough that the reader grasps, quite naturally, that all of them are serving the message of love.