Watching his mistress play the virginals (keyboard instrument), the poet envies the keys and wood she touches, wishing his lips could replace them.
The virginals (jacks, wood, wiry concord) are described in unmistakably sexual language—leap, kiss, tender inward, harvest, boldness, blushing, tickled, saucy. The instrument becomes a double metaphor for the woman's body and for male rivalry. 'Jacks' (the mechanisms) are personified as rivals who enjoy what the poet cannot. The poem turns musical performance into a theater of erotic jealousy.
The Dark Lady's fingers 'walk with gentle gait' across the keys, displaying control and grace. The poet contrasts her mastery of the instrument with his own powerlessness—his lips merely stand 'blushing' and unable to 'reap' the harvest. The poem reverses traditional gender roles: she is the performer with agency; he is the audience consumed by desire. This power dynamic shapes the entire Dark Lady sequence.
Like watching someone you're attracted to play video games or work on their laptop and feeling jealous of the controller in their hands. The poem channels that desire-adjacent frustration: wishing you were the thing that gets touched and receives her attention and technical precision.