In the lines you have cited from this love poem, the poet, E. E. Cummings, personifies the moon and the sun. The speaker of the poem says that "you are whatever a moon has always meant / and whatever a sun will always sing is you." Personification is the attribution of human qualities to something that is not human. Here, the moon has the power to mean; it has purpose and intent. Also, the sun is given the ability to sing. The speaker addresses his or her loved one, saying that the moon always means "you" and the sun always sings "you." Often, people in love or singers of love songs speak about the moon or the sun—how romantic the moon is when one is in love, how much their loved one is like the sun to them, or how the sun or stars seem to sing when one feels one is in love, and so on—and here, the speaker seems to nod to all such sentiments. To the speaker, his or her love makes everything seem alive and full of music; the sun and moon, the whole universe even, seem to speak of the narrator's beloved, because that is how in love he or she is.