Therefore the redeemed of the Lord - This is probably the language of Yahweh assuring them, in answer to their prayer, that his ransomed people should again return to Zion.
And everlasting joy shall be upon their head - This entire verse occurs also in Isaiah 35:10. See it explained in the note on that verse. The custom of singing alluded to here on a journey is now very common in the East. It is practiced to relieve the tediousness of a journey over extended plains, as well as to induce the camels in a caravan to move with greater rapidity. The idea here is, that the caravan that should return from Babylon to Jerusalem, across the extended plains, should make the journey amidst general exultation and joy - cheered on their way by songs, and relieving the tedium of their journey by notes of gladness and of praise.