When you are celebrating the Holy Eucharist, you may have observed at the time after the Rite of Peace when the Agnus Dei (Lamb of God) is being sung, the celebrant breaks a piece of the Bread and puts it into the chalice. What does this symbolise? The priest breaks the Eucharistic Bread, assisted, if the case calls for it, by the deacon or a concelebrant. Christ’s gesture of breaking bread at the Last Supper, which gave the entire Eucharistic Action its name in apostolic times, signifies that the many faithful are made one body (1 Cor 10:17) by receiving Communion from the one Bread of Life which is Christ, who died and rose for the salvation of the world. The fraction or breaking of bread is begun after the sign of peace and is carried out with proper reverence, though it should not be unnecessarily prolonged, nor should it be accorded undue importance. This rite is reserved to the priest and the deacon.The priest breaks the Bread and puts a piece of the host into the chalice to signify the unity of the Body and Blood of the Lord in the work of salvation, namely, of the living and glorious Body of Jesus Christ. The supplication Agnus Dei, is, as a rule, sung by the choir or cantor with the congregation responding; or it is, at least, recited aloud. This invocation accompanies the fraction and, for this reason, may be repeated as many times as necessary until the rite has reached its conclusion, the last time ending with the words dona nobis pacem (grant us peace).
Source : GIRM
