Psalm 72 is the last psalm in Book II (Psalms 42-72) of the Psalter. There is a doxology at the end of this psalm (verses 18-19), and a statement that at one time in the history of the development of the psalms, Psalm 72 was the last psalm in a smaller psalm collection (verse 20). So, actually, Psalm 72 consists of verses 1-17.
Psalm 72 is a prayer for the earthly king who reigns in Jerusalem or Zion, which was probably used, along with other psalms in the Psalter, at the coronation of each new king of Judah. The first paragraph in Psalm 72 is verses 1-7:
"Give the king your justice, O God,
and your righteousness to the king's son.
May he judge your people with righteousness,
and your poor with justice.
May the mountains yield prosperity for the people,
and the hills, in righteousness.
May he defend the cause of the poor of the people,
give deliverance to the needy,
and crush the oppressor.
May he live while the sun endures,
and as long as the moon, throughout all generations.
May he be like rain that falls on the mown grass,
like showers that water the earth.
In his days may righteousness flourish
and peace abound, until the moon is no more."
1. The fundamental characteristics of God's rule over all the earth are justice and righteousness (see Psalms 89:14, 16; 96:13; 97:2, 6; 98:9; 99:4; 146:7). Thus, naturally, the psalmist prays that the earthly king whom God has set up to rule his people in Zion will represent God faithfully by ruling with justice and righteousness.
2. God's particular concern is for those who cannot defend or support themselves: the "poor" and the "needy" (verses 2b, 4a-b, 12-13)--orphans, widows, and aliens (see Exodus 22:21-24; 23:6, 9; Deuteronomy 24:14-22; Isaiah 58:1-12; James 1:27). So, certainly, God expects his earthly king and his chosen people to share that concern, and to do all they can to help the poor and needy (see Acts 20:35; 1 John 3:17).
3. God expects those whom he blesses to share those blessings bountifully with others. Thus, the psalmist compares the king (whom God richly blesses) with rain which falls abundantly on the earth, causing the plants to grow and produce good fruit. In verse 7, these plants and fruit are righteousness and peace.
May God help us be genuinely grateful for all the ways he blesses us, and be generous in sharing his blessings with others.