What is Purgatory - Confraternity of Intercessors for the Souls in Purgatory

The Church is triumphant, purgative and militant. It is triumphant the part of the community that is already in Heaven. It is activist every member of the community who, like you, is still alive. And it is purgative the member of the community who is dead and is not in heaven, but in purgatory.

148.- Where are the members of the Church? - The members of the Church are partly in heaven, and form the Church triumphant; partly in purgatory, and form the Church purgative or patient; and partly on earth, and form the Church militant.
From the Larger Catechism of St. Pius X

All those who die in venial sin pass through purgatory.

1030 Those who die in the grace and friendship of God, but imperfectly purified, although they are sure of their eternal salvation, undergo purification after their death, in order to obtain the holiness necessary to enter into the joy of heaven.
St. John Paul II, Catechism of the Catholic Church,

790. Do those who die after receiving absolution, but before having fully satisfied the justice of God, go immediately to heaven? – No, sir; they go to purgatory, there to satisfy the justice of God and to purify themselves entirely.
From the Larger Catechism of St. Pius X

The souls in purgatory suffer greatly.

1031 The Church calls purgatory to this final purification of the elect, which is completely distinct from the punishment of the damned. The Church has formulated the doctrine of faith concerning purgatory above all in the Councils of Florence (cf. DS 1304) and Trent (cf. DS 1820; 1580). The tradition of the Church, with reference to certain texts of Scripture (e.g., DS 1820; 1580), has also been based on the doctrine of purgatory. 1 Co 3, 15; 1 P 1, 7) speaks of a purifying fire: "Concerning certain light faults, it is necessary to believe that, before the judgment, there is a purifying fire, according to what He who is the Truth affirms, when He says that if anyone has pronounced a blasphemy against the Holy Spirit, this will not be forgiven him either in this age or in the future (Mt 12, 31). In this phrase we can understand that some faults can be forgiven in this century, but others in the future century (St. Gregory the Great, Dialogi 4, 41, 3).
St. John Paul II, Catechism of the Catholic Church.

We members of the Church militant - you, if you are a Christian - can alleviate the sorrows of the souls in purgatory.

Can we relieve the souls in purgatory of their sorrows? - Yes, sir; we can relieve the souls in purgatory of their sorrows with prayers, alms, all kinds of good works, indulgences and, above all, with the holy sacrifice of the Mass.
From the Larger Catechism of St. Pius X

Because charity unites the one church, triumphant, purgative and militant.

224.- Does the communion of saints also extend to heaven and purgatory? – Yes sir; the communion of saints extends also to heaven and purgatory, because charity unites the three Churches: triumphant, purgative and militant; the saints pray to God for us and for the souls in purgatory, and we give honor and glory to the saints, and we can relieve the souls in purgatory by applying masses, alms, indulgences and other good works in suffrage.
From the Larger Catechism of St. Pius X

You also have obligation to pray for the souls in purgatory. It is an obligation of charity, and therefore especially important.

279.- For whom should we pray? - We are to pray for everyone, namely: for ourselves, for our parents, superiors, benefactors, friends and enemies; for the conversion of poor sinners, of those outside the Church and for the blessed souls in purgatory.
From the Larger Catechism of St. Pius X

Those of us who are still on earth can do much to alleviate the sorrows of those in purgatory. In particular, we can TAKE THEM OUT OF PURGATORY by offering them a plenary indulgence.

807. Can indulgences also be applied to souls in purgatory? - Yes, sir; indulgences can also be applied to the souls in purgatory, provided that the one who grants them declares it.
From the Larger Catechism of St. Pius X.

In other words, YOU have an OBLIGATION of charity towards the souls in purgatory, which consists in alleviating their suffering by gaining each day a plenary indulgence and offering it for a soul in purgatory.