Roman Catholicism has taught to pray for people who died before us. The Old Testament even has alms and prayers for those who died. A person that dies most likely will spend some time in purgatory. The length of time depends on what God deems fair.
What is Purgatory?

Purgatory is a place where souls become perfected. Since God is perfect, souls should be in a clean state before being physically with God. Luckily, unlike Hell, purgatory is not eternal. The length of time spent in purgatory is individualized, but we will be with God when we’re finished.

Purgatory may be as long as centuries in the case of the more guilty souls, or of those who, being excluded from the Catholic communion, are deprived of the suffrages of the Church, although by the divine mercy they have escaped Hell.

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If you read about Mary in Fatima, she was asked if an 18 year old girl was in Heaven – Mary said she would be in purgatory until the end of time.
When we’re alive, we can change our fate. By prayers, especially the Rosary, being good, receiving the Eucharist, and helping others we can probably reduce our time in purgatory. It all comes down to how pure our soul is.

At the end of the world, purgatory also ends. God will reconcile His justice in the purification of the last members of humankind. Then God will make His final judgement of Heaven or Hell. This judgement will be permanent.

Prayers and Sacrifices

People who are alive have a huge responsibility. We need to pray for people who are in purgatory that they can be released soon into Heaven. We don’t want anyone to suffer. A soul cannot do anything to release themselves from purgatory. They do not have a body. Only those who are alive has the ability to pray for the dead.

Along with prayer, we can fast and dedicate our Eucharist to the dead. All these things are very important and will allow souls enter the kingdom of God sooner than expected.

Indulgences

The Catholic Church has something called indulgences. It’s very difficult to get a full indulgence, but we can get partial ones. This helps clear our souls.

A partial indulgence can be obtained by devoutly visiting a cemetery and praying for the departed, even if the prayer is only mental. One can gain a plenary indulgence visiting a cemetery each day between November 1 and November 8. These indulgences are applicable only to the Souls in Purgatory.

A plenary indulgence, again applicable only the Souls in Purgatory, is also granted when the faithful piously visit a church or a public oratory on November 2. In visiting the church or oratory, it is required, that one Our Father and the Creed be recited.

A partial indulgence, applicable only to the souls in purgatory, can be obtained when the Eternal Rest (Requiem aeternam) is prayed. This is a good prayer to recite especially during the month of November:

Eternal rest grant to them, O Lord, and let perpetual light shine upon them. May the souls of the faithful departed, through the mercy of God, rest in peace. Amen.