The Portiuncula or small portion refers to the land where an old church dedicated to the Virgin Mother of God stood. It belonged to the Benedictines but was abandoned. St. Francis who has great devotion to the Blessed Mother began to live there and repaired it.  He heard that Angels often visited it so it was known as St. Mary of the Angels.

The Indulgence:

The "Pardon of Assisi" or "Portiuncula indulgence" is the first plenary indulgence that was ever granted to a Church. St. Francis himself petitioned Pope Honorius III after the Lord Jesus Christ and the blessed Mother appeared to him with a multitude of radiant Angels while he was praying at the Portiuncula. The Pope was astonished and was hesitant at first. It was only after he had come to the conviction that Jesus Christ himself wished it, that he granted the petition of the saint and confirmed the indulgence 

The Portiuncula Indulgence could at first be gained only by visiting the Portiuncula chapel between the afternoon of August 1 until sunset of August 2. Later Popes extended the privilege to all Franciscan churches and to all Parish churches

August 2 is the feast of Our Lady of the Angels of Portiuncula. A plenary indulgence is available  to anyone who will:

1. Receive sacramental confession (8 days before or after)

2. Receive the Holy Eucharist at Holy Mass  from afternoon of August 1 to midnight of August 2 (Preferably in a Franciscan church or Parish church)

3. Enter a Franciscan church or Parish church and, with a contrite heart, pray the Our Father, Apostles Creed, and a prayer of his/her own choosing for the intentions of the Pope.

Note:

It is important that the faithful must be free from all attachment to sin, even the slightest venial sin. Failure to meet them will effect no plenary indulgence but only partial indulgence.

Please tell every baptized Catholic person you know that remission of the punishment for all sins committed and confessed from the day of baptism to the reception of the indulgence is available. It can be applied to oneself or to a departed soul.