In just a few words, this short poem pictures a quiet moment in bed, where the sound of the sea seems to fill the room. The speaker says we "borrow" a great music from the sea, as if the waves lend us their rhythm for a while. It's a gentle idea: the ocean's sound isn't ours, but we can take it in and let it settle around us.
The last lines, "and so still / the bed," show the effect. The bed becomes calm and motionless, almost as if the sound has soothed everything in the room. The title, "Composed," works in two ways. It hints at music being composed, and it also means being calm and at peace. The speaker seems content, even grateful. The mood is hushed and restful, the kind of stillness you feel when you're listening closely and have no wish to move. The poem's brevity adds to this, leaving lots of silence around its few words.