The Holy Souls
They are the souls in Purgatory who are going through purification. But Purgatory is not just a waiting place for the Holy Souls to go to Heaven, because they do suffer, you may seen burning fire in a lot of images which artists portrayed Purgatory, but how is it like? As St John Henry Newman's poem mentioned 'the flame of Everlasting Love doth burn ere it transform...', it also said the two most painful sufferings for those poor souls are the longing and yearning for God but could not see Him; and at the same time the shame of self at the thought when seeing Him because their being as been so vile and want to hide themselves from His sight. The period in Purgatory is different for each Holy Souls, it could be days, months or even years. And the worse thing is, they can no longer help themselves as they cannot gain any merits once their lives ended. However, we could help them to leave Purgatory earlier by offering our prayers for them, and the best prayer is the Holy Mass - the Eucharistic Sacrifice. We are also encouraged to almsgiving, gain indulgences and do works of penance on their behalf. What is more, Our Lord is also very pleased that we do so, because His Love urges Him to release the Holy Souls. Jesus once explained to St Gertrude that if a merciful king's friend was found guilty and sent to prison because of justice, the king will be longingly waiting for a noble to beg for him and give some offerings for his freeing. So the king will then happily, release him. Likewise, Our Lord is extremely pleased to accept what is offered to Him for the Holy Souls, since He has such great desire to have those souls whom He has paid with such a great price to be close to Him again.
(Sources & References: The Dream of Gerontius, St John Henry Newman, 1865. Catechism of the Catholic Church, Paragraphs # 1032 http://www.catholictradition.org/Gertrude/saintgertrude7.htm)