Prayer said by the celebrant, reading from the altar missal, at the end of the Communion Rite.
Part of the Proper of the Mass and therefore changes with each mass. The appropriate Post Communion for a given day is found in DWM (and should be marked with the red ribbon).
Said after the Prayer of Thanksgiving.
Then the Priest says:
Let us Pray.
Then the Priest says the Postcommunion Prayer, at the end of which the People acclaim: Amen.