Holy water is just water mixed with a little bit of salt. Both the water and the salt have been blessed by an ordained person. Holy water is called a sacramental and it has sanctifying properties only if the user has true faith. It is just another tool to accept God’s graces, but you better have real faith. Some people that do not understand the proper use of sacramentals use it in a very superstitious way and that is condemned by the Church. If you are interested in having a better understanding of Holy Water and sacramentals in general I encourage you to read the following link:
Sacramentals