The sacristy is the room where the priest prepares for the celebration of Mass.
In each Catholic church, there exists a room where the priest will prepare for the celebration of Mass.
This room is called a sacristy, which comes from the medieval Latin word sacristia, from the same root as “sacred.”
The Catholic Encyclopedia summarizes what is contained in this room.
A room in the church or attached thereto, where the vestments, church furnishings and the like, sacred vessels, and other treasures are kept, and where the clergy meet and vest for the various ecclesiastical functions … It is customary to have a holy water font, and a bell to admonish the congregation of the advent of the clergy, at the door leading to the sanctuary. The sacristy is not blessed or consecrated together with the church, and consequently is not a sacred place in the canonical sense.
If you ever hear a Catholic talking about the “sacristy,” now you know what it is!