Two people are far apart, and the speaker reaches toward the one who is away. The poem begins with a wide view of the horizon and the sky, where distant music seems to rise and fall from the same stars that shine on both of them. That shared sky is the speaker's proof that the distance isn't total.
From there, the poem turns to the act of writing. The speaker imagines the two of them as ships, "together and alone," slipping free of the space that keeps them apart. The pen becomes a way of drawing a route toward the other person's quiet, comfortable world. In the closing image, the light from the speaker's window spreads like a sail and drifts into the other's "lambent," or softly glowing, harbor.
The mood is tender, calm, and a little wistful. The speaker longs for closeness but doesn't sound desperate. There is a gentle hope that words, like light, can travel the distance and arrive.