Immortality

The speaker talks about Jove's "nectar" (Jove a.k.a. Jupiter a.k.a. Zeus is the immortal king of the gods in Greek/Roman mythology) and expresses a desire that the "rosy wreath" (9) he sends Celia will not die in her presence. He even talks about how the soul requires a "divine" (6) drink – not Jove's nectar, but something equally out-of-this-world. By the end of the poem, he even implies that the "rosy wreath" continues to "grow," even though we know that that is impossible (a wreath is made of flowers or leaves that have been plucked, so they're dead). The implication is that the woman's breath is able to make things immortal.

• Line 6: The speaker says the soul requires a "drink divine." No, he isn't talking about a literally "divine" beverage (no heavenly Mountain Dew), but rather a show of love from Celia, or something similar. "Divine drink" is a metaphor here. Also, notice how the "d" sound is repeated at the beginning of two words? That's called alliteration.
• Line 7: The speaker mentions Jove, the king of the Greek and Roman gods (who were, you know, immortal). Nectar is the drink of the gods. The word nectar actually comes from two Greek words that combined mean something like "overcoming death" (nekros, dead body; tar, overcome).
• Lines 11-12: The speaker sends Celia a wreath in order to see if it will live forever (i.e., become immortal) in her presence. The wreath is here a symbol of artistic creation itself.
• Line 15: The speaker says the wreath continues to "grow," which suggests that it is immortal. This is a bit odd because you can't make a wreath without killing the leaves or flowers it's made of, so the fact that the wreath still grows suggests that the speaker's experiment has succeeded. The wreath's continued growth is a symbol of immortality and is also meant to symbolize the speaker's hopes for the continued life of his relationship to Celia.