The psalm begins with an invitation: come, bless the Lord, all you servants of the Lord, who stand by night in the house of the Lord! It refers to the priests and Levites who served during the night watches, guarding the temple and its offerings. To bless means to acknowledge and praise God for who he is; it is not giving something to God but expressing gratitude and honor.
Verse 2 shows how: lift up your hands to the holy place and bless the Lord. Lifting hands was a gesture of prayer and worship, of surrender and expectation. The sanctuary was the place where God chose to make his name dwell.
Verse 3 changes voice. Perhaps the priests answer, or a voice from the temple: may the Lord bless you from Zion, he who made heaven and earth. So the collection ends as Psalm 121 began, with the Maker of heaven and earth. The blessing comes from God to the one who praises him, and human praise is answered with divine blessing.