What is the significance of the rosy wreath in the poem "Song: To Celia"?

But might I of Jove's nectar sup, I would not change for thine.

Jove's nectar makes one immortal. But Jonson's speaker would rather have Celia's nectar to ensure immortality.

I sent thee, late, a rosy wreath, Not so much honouring thee, As giving it a hope, that there It could not withered be.

Celia's beauty is so amazing, Jonson's speaker argues, that it can give immortality. So he sends her a wreath of roses, not so much to honour her, but because it (and he) hoped that, by being near Celia it wouldn't die - it wouldn't wither. Because she has this power of immortality, hopefully the roses would live.

But thou thereon did'st only breath, And sent'st it back to me: Since when it grows, and smells, I swear, Not of itself, but thee.

Celia did simply breath on it, and sent it back to the speaker. Since she did that, the wreath is now growing, and smelling - not of itself (i.e. of roses) but of Celia.

Hope it helps!