The term guardian angels refers to the belief that each person has an angel who is available to shepherd their soul through life, and help bring them to God.
Belief in the reality of angels, their mission as messengers of God, and man’s interaction with them, goes back to the earliest times. Cherubim kept Adam and Eve from slipping back into Eden; angels saved Lot and helped destroy the cities of the plains; in Exodous Moses follows an angel, and at one point an angel is appointed leader of Israel. Michael is mentioned at several points, Raphael figures large in the story of Tobit, and Gabriel delivered the Annunciation of the coming of Christ.
The concept of each soul having a personal guardian angel, is also an ancient one, and long accepted by the Church
See that you despise not one of these little ones [children]: for I say to you, that their angels in heaven always see the face of my Father who is in heaven. – Jesus, Matthew 18:10The feast celebrating the angels who helped bring us to God began in many local calendars centuries ago, and was widely known by the 16th century. Pope Paul V placed a feast venerating the angels on the general calendar on 27 September 1608. Ferdinand of Austria requested that it be extended to all areas in the Holy Roman Empire. Initially placed after the feast of Michael the Archangel, it was seen as a kind of supplement to that date. Pope Clement X elevated the feast, celebrated 2 October, to an obligatory double for the whole Church. On 5 April 1883, Pope Leo XIII raised the feast to the rank of a double major.
How great the dignity of the soul, since each one has from his birth an angel commissioned to guard it. – Saint Jerome in his commentary on Matthew
Are they not all ministering spirits, sent to minister for them, who shall receive the inheritance of salvation? – Hebrews 1:14