Part II: The procession.

Before mass, the servers, celebrant in his vestments, and any other ministers line up for the procession. The servers may wear an alb, or a shortened version, a surplice. In either case, it is white, symbolic of purity. If they wear a surplice, they also wear a cassock underneath. it is usually, black, symbolic of dying to self, which is what the priest and servers are expected to do, especially in the mass, when their personalities should disappear under the vestments. In some places, they may change with the liturgical season, or according to the rank of the church. (Such as violet for a Cathedral church, or a monsignor's parish.)
In the procession for sung mass will usually be:
1) The Thurifer and Boat boy.
The thurifer has the job of carrying incense. Incense will be explained later, but for now, keep in mind that the use of incense was once commanded by God under the sacrifices of the old covenant. (Leviticus 2:2, 2:15-16, Luke 1:8-10) Originally, in order not to be confused with the jews or pagans, and in order not to be found by the authorities who persecuted them, the early church did not use incense. But after she was vindicated from the catacombs, she used it in order to show that it was her sacrifice, the true and only sacrifice of Christ which brings glory to God, and the remission of the sins of the world, not the sacrifices of the Jews or pagans. They were the type, the Church the antitype.
2) The Crucifer.
The crucifer has the job of carrying the cross in precession. The cross itself is a representation, and a symbol of Christ in our midst. Its main purpose is to serve as a reminder to the faithful to "Ponder nothing earthly minded", as the hymn goes, but to put your mind attentively on the mystery of our salvation, the sacrifice of the cross which is to be made present to us. The cross acts as a first standard of our king, Jesus Christ. It comes to tell us that Christ will soon become present on the altar, and in some sense, is a sign to us that he is present in the priest, as well as in all of us.
3)Acolytes with candles.
Candles of course, represent Christ, who said he was the Light of the World. On the altar, or on gradines behind it, we may see four or six lights lighted. Four are all that are necessary for a sung mass under the old rubric, but six are customarily lighted on feastdays, and every Sunday at many parishes. When there are four, they can remind us of the ffour living creatures of the book of Revelations, which ever and always gave themselves up to in worship to the glory of God and the lamb, as the candles give themselves up by being burned to the glory of God.
When there are six, they can remind us of the seven lamps which burn before the throne of God in heaven. ( Revelation 4:5)
4) The celebrant.
In the mass, the priest as we know acts in the name and person of Christ, in offering the holy sacrifice. Truly, it is not the priest who offers the mass by his own power, but Christ who does it. He consecrates the bread and wine of man into the bread of heaven and the chalice of salvation, his own body and blood. He offers himself to the father on our behalf. We respect the priest, ot out of clericalism, but out of respect for Christ who works through the priest. You will often see people bow to the celebrant as he walks in procession to the altar.

A lector, or additional servers may be in the procession, as well as a master of ceremonies, deacons and concelebrants in solemn mass.
Of the origin procession, Dom Gueranger says: "The Procession, which in many Churches immediately precedes a Solemn Mass, is a prelude to the great Aot which is about to be accomplished. It originated from the practice used in Monasteries, of going through the Cloisters, every Sunday, whilst chanting certain appointed Responsories; and during which, the Hebdomadarian went through all the Conventual Places, blessing each of them. The practice is still in use."
After the celebrant blesses the incense with the sign of the cross. The first of many to be made!) The sacristan or a server may ring the sacristy bell, and in a flurry of glory, the procession makes its way to the sanctuary. When it reaches the sanctuary, all genuflect, except the crucifer and acolytes, or anyone who may be carrying something. (The first of many genuflections.) The mass will begin with the introit.