The practice of exposing the sacred Host for the faithful to adore is an ancient practice in the Church. When Jesus gave us Himself in the Eucharist, He wanted His followers to know that "I am with you until the end of the age." When we are in a church and see the tabernacle light, we know He is there. Making His Body, in the form of bread, fully visible reinforces His promise of communion.
Adoration usually begins after a Mass. The celebrant places a large host in a vessel called the monstrance (from the Latin monstrare, "to show"). If adoration is to take place in the church, he will place the monstrance in the center of the altar, facing the people. If adoration is take place in a separate chapel, he will carry the Host in procession to that place. When carrying the monstrance the priest will wear a special vestment, called the humeral veil, which covers his hands as he holds the monstrance.
When the Host is exposed, it is traditional to kneel on both knees before entering a pew or chair and upon leaving.
If the Host is to be exposed for a lengthy period, individuals are asked to serve as "guardians." This usually means spending one hour in the church or chapel, praying or reading holy materials.