They pray for the outpouring of the full blessing which their High Priest, Jesus, is to bestow by their means on all the earth.
The language of verse 1 refers to Numbers 6:24,25, and very appropriately; for the time is the Lord's Second Coming, when, as true High Priest, he comes forth from the Holiest to bless the people.
The "Selah" at the end of verse 1 and verse 4 is, in both cases, very expressive, indicating, as it does, pauses in the sense and feeling, as well as the music.
"God be merciful to us
And bless us!
And cast the light of his countenance
(So as that it may be) with us."
Bless us and guide us in your way, your mode of dealing with your people, that by us your way may be known on the earth, as foretold in Genesis 12:3, and since those days in Amos 8:14; Isaiah 60:1,2; Acts 15:15-17; Romans 11:15, and many others places
"The nations shall praise you, O God;
The nations shall praise you." (v3)
The peculiar people here anticipate with joy the time when the whole Gentile people shall praise their God and Saviour, and that through their means.
"Let the tribes (of earth, those who once raged against you - Psalm 2:1) rejoice and sing,
For you judges (i.e. ruled) the nations righteously,
And as for the tribes of the earth, you guide them" (Isaiah 53:11, Hengstenberg)
And again at the happy prospect they cry, "Hallelujah!" for they repeat their song -
"The nations shall praise you, O God,
The nations shall praise you! Every one of them!" (v5)
And now Earth, as well as Palestine, gives its increase, for the curse is away, and the blessing rests on it (Lev 26:4). Israel rejoices in this communication of their blessing to all men -
"Earth gives its increase!
God, our God, blesses us!
God blesses us!
And they fear Him!
All ends of the earth!"
Horsely says, this is "A hymn for the Feast of Tabernacles, prophetic of a general conversion of the world to the worship of God." Dr. Allix entitles this Psalm, "A prayer of the Synagogue for the Second Coming of the Messiah, when her empire is to be extended over all nations,and the temporal blessings which are promised to the Jews in several oracles shall be conferred on them."
But it is simpler, and perhaps more correct to describe it - The Prayer of Israel for the blessing which Messiah is to give them, for the sake of the earth at large.