92 In this most important subject it is necessary, in order to avoid giving rise to a dangerous error, that we define the exact meaning of the word "offer." The unbloody immolation at the words of consecration, when Christ is made present upon the altar in the state of a victim, is performed by the priest and by him alone, as the representative of Christ and not as the representative of the faithful. But it is because the priest places the divine victim upon the altar that he offers it to God the Father as an oblation for the glory of the Blessed Trinity and for the good of the whole Church. Now the faithful participate in the oblation, understood in this limited sense, after their own fashion and in a twofold manner, namely, because they not only offer the sacrifice by the hands of the priest, but also, to a certain extent, in union with him. It is by reason of this participation that the offering made by the people is also included in liturgical worship.
Source: Mediator Dei (Vatican.va)