Expectation of the Messiah and his Spirit
The People of the "poor" - those who, humble and meek, rely solely on their God's mysterious plans, who await the justice, not of men but of the Messiah - are in the end the great achievement of the Holy Spirit's hidden mission during the time of the promises that prepare for Christ's coming. It is this quality of heart, purified and enlightened by the Spirit, which is expressed in the Psalms. In these poor, the Spirit is making ready "a people prepared for the Lord."
Source: Catechism of the Catholic Church (Vatican.va)