The title borrows the language of a doctor's note, and the poem plays along: it reads like a list of ingredients for a cure. What the speaker and a companion take "for love and quiet" is oddly simple: a mouthful of age, a single night, and the sound of rain. "A mouthful of age" is the strangest of these. It might suggest the taste of growing older, or the slow, savoring pace that comes with it, though the poem leaves that open.
Nothing dramatic happens. The couple seem to lie in the dark, listening to rain, drifting in the space "between waking and sleeping." That in-between state is where the speaker finds a feeling they've never had so fully before: "so much content." The word can mean contentment, and the plain, almost understated phrasing makes the feeling believable rather than showy.
The mood is hushed, grateful, and gently tender. The poem suggests that peace doesn't need much, just a night, some rain, and each other.