Catholics use “holy water” for a rather obvious reason. Water is used by all people to cleanse ourselves. In a religious setting (not only for Catholics, but for many other religious folks: Muslims, Buddhists, other Christians), water is used as a symbol for ritual cleansing of sin.
This first occurs in the Sacrament of Baptism. Catholic churches have small fonts (dishes) at their entrances, filled with “holy water” (water blessed by priests) and will dip their fingers in the water and make the sign of the cross (touching their foreheads, chest, left shoulder and right shoulder, “in the name of the Father, the Son, and the Holy Spirit”) to recall and renew their baptismal ritual. Holy water is also used to cleanse the community gathered for Mass at certain occasions, when the priest will sprinkle the water over the congregation, citing the words from Psalm 117, “Sprinkle me, O Lord, with hyssop and I will be clean; wash me and I will be whiter than snow.”