Wikipedia
The altar of repose is an altar where the Communion hosts consecrated on Maundy Thursday during the Mass of the Lord's Supper are placed, or " reserved", for use on the following day, Good Friday. The altar can be found in Roman Catholic, Anglican (especially Anglo-Catholic), and some Lutheran churches. Good Friday is the day on which the death of Christ is observed. His Resurrection is not observed until Easter Sunday and the anticipatory Easter Vigil on Holy Saturday. Between the time of his death and resurrection, mass is not celebrated, and so communion hosts cannot be consecrated. Any hosts used on Good Friday must have been consecrated previously
The Roman Catholic Church does not require that the place of reservation should be an altar, only that "the Blessed Sacrament should be reserved in a closed tabernacle or pyx". Indeed the Church's rules on the matter envisage no more than a single altar in the church.
In the Mass of the Lord's Supper, a sufficient number of hosts are consecrated for the faithful to receive Communion both at that mass and on the next day, Good Friday. The hosts intended for the Good Friday service are not placed in the tabernacle, as is usual, but are left on the altar, while the priests says the postcommunion prayer. They are then carried in solemn procession to a place of reservation somewhere in the church or in an appropriately adorned chapel. The priest uses a humeral veil while carrying them to that place. The procession is led by a cross-bearer accompanied by two servers with lighted candles; other servers with lighted candles follow and a thurifer with incense immediately precedes the priest. At the end of the Holy Thursday service, all altars, except the one used as the altar of repose, are stripped. The Blessed Sacrament remains in that temporary place until the Holy Communion part of the Good Friday liturgical service.
Roman Catholic piety has made Maundy Thursday a day of exceptional devotion to the Blessed Sacrament, and the place where the Sacrament is reserved is a focus for the love and aspirations of the faithful. Eucharistic adoration is encouraged at the place of reservation, but if continued after midnight should be done without outward solemnity. In many urban cities, the practice has developed among the faithful of traveling from one church to another to pray in front the different church's Altar of Repose, a practice called Seven Churches Visitation. In the Philippines, this practice is called Visita Iglesia.
At the Good Friday service (The Celebration of the Passion of the Lord), the Blessed Sacrament is available for Communion. After that service (with the altar of repose being dismantled), it remains available as viaticum for the dying in a less conspicuous location such as a locked cabinet in the sacristy. While the receptacle remains in such a temporary tabernacle, a lamp or candle is kept burning before it.
Mention of the altar of repose and the procession to it is not found before the close of the fifteenth century. The reservation of the Consecrated Species in the Mass of Holy Thursday, spoken of in earlier liturgical works, was for the distribution of Holy Communion, not for the service on the following day.