Immediately after the First Reading of the Liturgy of the Word at Mass comes the Responsorial Psalm. While its name suggests it’s a response to the First Reading, actually it’s an integral part of the Liturgy of the Word in its own right. It does often develop a theme or idea that is observable in one of the readings, but not always. Sometimes its use on a particular day is seasonal or traditional.
The Responsorial Psalm is called by that name because of its structure, not its location in the order of worship. The cantor or choir leads it, with the entire congregation repeating a refrain (the “church” word for this is antiphon!) in between the verses of the psalm. In this way the active participation of the congregation is encouraged in an external way – and not just in an internal way, as occurs frequently at other times during the Mass.
As a rule, the Responsorial Psalm is taken from the Lectionary for Mass, although for a good reason it is possible to substitute another psalm or Scriptural song. Only if it cannot be sung is the psalm recited, but still always in a responsorial way.