The Chrism Mass, ordinarily celebrated on Holy Thursday morning by the Bishop and concelebrated with the priests of the diocese, is the Mass when the bishop consecrates the holy chrism and blesses the oil of catechumens and the oil of the sick. Priests are brought together and concelebrate this Mass as witnesses and cooperators with their bishop in the consecration of the chrism because they share in the sacred office of the bishop in building up, sanctifying, and ruling the people of God. This Mass is therefore a clear expression of the unity of the priesthood and the sacrifice of Christ, which continue to be present in the Church. The Mass takes its name from the most eminent of the three holy oils which the bishop commissions for his local church's use over the following year.
While the Oil of the Sick, used for those who seek the anointing, and the Oil of the Catechumens, which is imposed on those preparing for baptism, are simply "blessed," the Sacred Chrism is "consecrated," and all the priests present participate in the latter moment by extending their hands toward the vessel containing it as the bishop says the prayer of consecration.
The Chrism is used at the ordination of priests and bishops, baptisms, confirmations, the consecration of altars and the blessing of churches, where the walls are smeared with it in the shape of the sign of the cross.
As part of the consecration of the Chrism, balsam is poured into the oil, which gives it a sweet smell intended to remind those who encounter it of the "odor of sanctity" to which those people and things who are marked with it, and by extension all of us, are called to strive for.