Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
This is the last psalm of the collection or group called "Songs of Degrees," and it is of the nature of a doxology as now sung in our places of worship. Its author is unknown. From anything that appears in the psalm itself, it may have been composed originally to occupy the very place which it does occupy here. The psalm is a summons to praise, and it would seem not improbable that it was designed to be sung by alternate choirs - the first Psa 134:1-2 representing the people approaching the sanctuary, calling on those who habitually serve God there - the ministers of religion - to lift up their hands in the sanctuary and to praise the Lord; the second Psa 134:3, the response of the priests or the ministers of religion, pronouncing a blessing on the people - a blessing as proceeding out of Zion.
Behold - As if calling attention to the fact that they were there, or had come.
Bless ye the Lord - Praise Yahweh. Making known their desire that God should be praised, and calling on those who presided over the public worship of the sanctuary to engage now in that service as expressive of their feelings.
All ye servants of the Lord - The priests or ministers of religion, appointed especially to this service.
Which by night stand in the house of the Lord - There was a class of singers in the temple who devoted the night, or a part of the night, to praise; and it is possible that this service may have been, as it was subsequently in some of the monasteries, continued by succeeding choirs, during the entire night. Thus in Ch1 9:33, it is said, "And these are the singers, chief of the fathers of the Levites, who remaining in the chambers were free, for they were employed in that work day and night." This class is particularly addressed in this psalm, as if they were especially favored, or as if they especially possessed the ear of God in the silence of the night, and when the world slumbered around them. There is something favorable to devotion in the silence of the night; when the world sleeps; when we are alone with God; when it seems as if God would more particularly attend to our cry since the rest of the world is still, and does not (as it were) need his care. All this may be fancy; but the effect may be to make the mind more solemn, and better suited for devotion.
Lift up your hands in the sanctuary - Margin, In holiness. The Hebrew word properly means holiness, but it may be applied to a holy place. See Psa 20:2. The lifting up of the hands is properly expressive of prayer, but the phrase may be used to denote praise or worship in general.
And bless the Lord - In the night-watches - while all around is still, - let there be one place where the voice of praise shall ascend on high.
The Lord that made heaven and earth - The great Creator of all things. This is probably the language of those who were thus employed in the service of the Lord at night; their response to the address in the first two verses.
Bless thee out of Zion - That is, bless those who thus approached the sanctuary, and called on those within to praise the Lord. This is the answer. Let the blessing of God rest on you. It is language showing that they appreciated the kind and encouraging salutation, and that they reciprocated the feelings and the good wishes of those who came to worship. In the name of the Lord whom they served, therefore, and appealing to him, they pronounced a blessing on those who thus approached the sanctuary. People do not come near the house of God - the place of public worship - with kind and sympathizing feelings without a blessing from the sanctuary, without a response that welcomes them, and that meets all their aspirations. There is always in Zion - in the church - a voice, by day and night, which pronounces a blessing on those who wish it well, who seek its good, and who desire to partake of the favor of God.
Out of Zion - That is, may God speak to you out of Zion; may he confer on you such blessings as properly go out of Zion; or such as Zion (or his church) can furnish. Go not away unblessed; go not without a token of divine favor - for God will bless you.