Three short lines capture a moment of intimacy between two lovers, and the title suggests we are overhearing something private. The speaker is looking at his lover's mouth and finds it "pretty as night," a surprising comparison that makes her beauty feel dark, soft, and a little mysterious. Then the image shifts: her mouth becomes a rose, and he locks his own mouth to it, as if kissing is both a delicate act and a kind of holding fast.
The last line, "saying her fill," is the most interesting. It can mean she is speaking everything she wants to say while he kisses her, or that she is taking her fill of him through the kiss. The "conversation" of the title may be carried by mouths meeting rather than by words. The mood is hushed, sensual, and tender, with a feeling of complete absorption in the other person. Because the poem is so brief, it feels like a single breath or a stolen kiss.