When you walk into a Church, you look for the holy water to cross yourself. When you arrive at the pew, you genuflect to the Blessed Sacrament in the Tabernacle. When someone comes over to speak to you in Church, you whisper. When you hold your rosary in your hand, you are gentle with it, being careful not to drop it. Or do you? If these are not your responses, then you can be pretty sure that you have lost (or maybe never had) a sense of the Sacred. For something to be sacred, means that it is separated from common use, and is reserved for something special.
Sacred vessels are the chalice, paten and ciborium that are used for holding and distributing the Eucharist. It would be sinful to use them for a common meal because they have been taken out of the area of common usage and dedicated for holy things. We are supposed to recognize this special usage, and respond rightly to it. Holy places, images, books, and various other objects are supposed to be distinct from what we use in our ordinary lives and are supposed to be treated in a distinct way.
This means that you are not supposed to use a crucifix (whether blessed or not) as a paper weight. A newspaper should not be placed on top of a Bible or a Catechism. An image of the Blessed Virgin should not be packed away in a box and forgotten. It also means that certain words should be spoken with reverence and never flippantly or casually; certainly the names of God, but also the Saints and other holy things. A home altar should not be treated as another piece of furniture when it not being used for prayer or devotion.
We could go one step further though, and acknowledge the holiness of certain things by showing a physical sign of reverence toward them. The old tradition of bowing the head at the name of "Jesus Christ" or "Christ Jesus" is a practice that should be retained (both inside the Mass and in everyday life). I have the habit of saying to myself, "blessed Jesus", every time I hear someone say the Lord's name in vain. Some today still acknowledge the presence of the Sacrament by crossing themselves when they pass in front of a Church (while driving or walking). There are also some people who still kiss the hands of a priest to give thanks for the ordination he received and the many things that a priest's hands do for the people.
These are all signs of recognizing things that are sacred and giving proper honor to them. There are many things in our lives that are sacred, and if we have become callous towards them it will impact our spiritual health. If you are callous toward sacred things, you will eventually become callous toward the One Who is Sacred in Himself. To go about our days acknowledging the sacred may seem--if you do not already do it--as a burdensome task. Yet, once the habit is formed you find that recognizing God's hand in your life is a great blessing.
Many today (sadly, even some clergymen) have encouraged a disrespect for God and sacred things. This disrespect will always lead to fear, isolation, confusion, and depression; when we move away from God, we move away from joy--always. So when your priest encourages you to acknowledge the awesome nature of the sacrifice of the Mass, and the great blessings of the Eucharist, he is showing you the path to holiness, peace, and joy. He is reminding you that the Sacred is a wonderful blessing in our lives. The existence of the Sacred means that we do not live in a pagan world; we live in a world created by God that is being redeemed by our Lord Jesus Christ (did you bow your head when you read His Name?).
