Horsebalm is a plant, and the title tells us what the speaker has probably picked up or brushed against: a dried, shriveled leaf. The poem is only three lines long, but it holds a small surprise. We're told it is three days from the solstice, which, judging by the shriveled leaf, suggests the year is at its darkest and the growing season is long over. Yet the leaf still smells of lemon.
The speaker doesn't make a big statement about this. The word "faintly" keeps the feeling quiet and a little tentative, as if the scent might vanish if you looked too hard. What comes through is a gentle pleasure, mixed with a sense of how fragile it is. Something dead-looking still carries a trace of life and brightness.
The mood is calm, attentive, and quietly hopeful. The solstice marks the turning point toward longer days, so the lingering smell can feel like a small promise, though the poem leaves that for the reader to sense.