All the people are called upon to bless the LORD, that is, to praise him (see 16.7). There are four groups mentioned: first, the house of Israel, that is, all the “people of Israel”; then the “priests” (house of Aaron); then the “Levites” (house of Levi); and finally all the worshipers (You that fear the LORD); see 115.9-11; 118.2-4, where the Levites are not mentioned by name. The Levites assisted the priests in the Temple services; among other duties they served as musicians and guards.

The Hebrew text in verse 21a has “Blessed be Yahweh from Zion,” which, as Kirkpatrick points out, corresponds to 134.3, “May Yahweh bless you from Zion.” The thought here is that from Zion (Jerusalem, or the Temple), where the worshipers have come together, praise must be offered to Yahweh. Since both the worshipers and Yahweh are in Zion, the easiest way to represent the meaning of the text is to say simply “in Zion.”

For the concluding Praise the LORD, see 104.35.

Quoted with permission from Bratcher, Robert G. and Reyburn, William D. A Handbook on the Book of Psalms. (UBS Helps for Translators). New York: UBS, 1991. For this and other handbooks for translators see here .