Someone sits under an umbrella and listens to the rain, and the poem turns that ordinary sound into something stranger. The rain on the fabric becomes "a great throng," a crowd of voices pressing in close to the ear. The speaker then pushes the idea further: the crowd could pass for "unseen diaries," as if each drop carried a private thought or confession that can't be read, only heard as a murmur.
The last two lines shift the focus inward. The speaker's mind is "a foreigner," an outsider who doesn't quite belong among all this noise, and it is "troubled by rogue citizens," which could mean the stray, unruly thoughts the rain stirs up. The mood is quiet but a little uneasy. The umbrella shelters the speaker from the weather, yet not from the crowded feeling of their own mind. The rain starts as a soothing sound and ends up feeling like an intrusion of voices.