Behold the Lord hath proclaimed - Proclamation is made to all nations that Yahweh is about to come and rescue his people.
Say ye to the daughter of Zion - To Jerusalem (see the notes at Isaiah 1:8).
Thy salvation cometh - Lowth renders this, ‘Lo!, thy Saviour cometh.’ So the Vulgate, the Septuagint, the Chaldee, and the Syriac. The Hebrew word properly means salvation, but the reference is to God as the Deliverer or Saviour. The immediate allusion is probably to the return from Babylon, but the remote and more important reference is to the coming of the Redeemer (see the notes at Isaiah 40:1-10).
Behold, his reward is with him - See these words explained in the notes at Isaiah 40:10.