Holy Orders

The sacrament of Holy Orders creates a priest.
There's a little more to it than that, of course. As the Catechism's section on Holy Orders says: this "is the sacrament through which the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry. It includes three degrees"—the orders of bishop, priest, and deacon. (Catechism, 1536)

But to keep things simple, let's start with the priest.

To know what a priest is we have to know what a sacrifice is.
Nowadays the word "sacrifice" is used in many different ways. But in its strict meaning, its original meaning, a sacrifice is the offering of a gift to God by a group, through the agency of someone who has the right to represent the group.

The purpose of such an offering is to give group worship to God; that is, to acknowledge God's supreme lordship over mankind, to thank him for his blessings, to atone for human sin, and to beg for his benefits.