What Does it Mean to Pray for the Poor Souls?

It means everything that we can offer for the faithful departed.

~We can offer our bodily pains in expiation for their sins.
~We can offer our spiritual sufferings, our disappointments and fears, our discouragement and estrangement from those we love.
~We can offer our vocal prayers, like the Rosary, the Memorare, the Angelus, the recitation of the Divine Office.
~We can offer our mental prayers, like the Way of the Cross, our daily meditation and examination of conscience.
~We can offer our mortifications, like giving up some delicacy at table, or performance of some unpleasant work.

But the most effective offering we can make for the Poor Souls is the Holy Eucharist at the Sacrifice of the Mass, Holy Communion and adoration of the Blessed Sacrament. Over the centuries, the Catholic faithful have by now offered countless Masses for the Poor Souls. In the Society of Jesus, we priests are expected to offer one Mass each month for all the deceased Jesuits. This amounts to over sixteen thousand Masses that are to be said every month for the members of the Society of Jesus who are still in purgatory.

May I offer a recommendation? During the month of November, I suggest that we make a list of all the deceased persons whom we wish to specially remember in our Masses, prayers and sacrifices for the repose of their souls. Add to this list as those enter eternity whom you wish to specially commend to the mercy of God. This, by the way, is called a Necrology. Every Catholic diocese in the world has a Necrology of its deceased priests. Every family should have its own Necrology of deceased members whom we daily remember to our merciful Lord.

Every time you say the grace after meals, be sure to add the invocation, "May the souls of the faithful departed, through the mercy of God, rest in peace. Amen." In every Rosary you recite, do not forget to say after each decade, " O, my Jesus, forgive us our sins, save us from the fires of hell, and bring all souls to heaven, especially those who are in most need of thy mercy."
Remember that devotion to the Poor Souls is really a covenant between them and us. We pray and sacrifice for them, They can pray and suffer for us. They appreciate whatever help we give them, to lessen their suffering and to shorten their stay in Purgatory. They will continue to show their appreciation when we join them in a heavenly eternity.

(From a homily by Fr. John A. Hardon, S.J. accessed here)