O give thanks unto the Lord, for he is good - This whole verse is the same as Psalms 106:1, except that that is introduced by a Hallelujah. See the notes at that verse.
For his mercy endureth for ever - See also Psalms 106:1, note; Psalms 107:1, note. Literally, “For unto eternity his mercy.” That is, It is ever the same; it never changes; it is never exhausted; it is found in all his dealings - in all his acts toward his creatures, and ever will be.