Vespers, also called Evening Prayer, is part of the Liturgy of the Hours, also known as the Divine Office. In the Liturgy of the Hours, the Church fulfills Jesus’ command to “pray always” (Luke 18:1; see also 1 Thessalonians 5:17). Through this prayer, the people of God sanctify the day by continual praise of God and prayers of intercession for the needs of the world.
Bishops, priests, deacons, and many men and women in consecrated life pray the Liturgy of the Hours each day. Their work is organized around this prayer, keeping God always at the center of their days. Lay people are encouraged to pray the Liturgy of the Hours as well, especially Morning and Evening Prayer.
Evening Prayer gives thanks for the day just past and makes an evening sacrifice of praise to God (see Psalm 141:1).
(Source: USCCB website)