This prayer is recited after meals, and at other times. It is a prayer for the holy souls in purgatory. It is a great act of kindness to pray for their relief.
May the souls of the faithful departed, through the mercy of God, rest in peace. Amen.
Fidelium animae, per misericordiam Dei, requiescant in pace. Amen.
Here is our Memrise course which will teach you this prayer in Latin. Review it five minutes daily for a week or two and you will be an expert.
Pray with understanding. Comfort the suffering souls. And, pray in Latin!