Gregorian Chant, according to the church document from the Second Vatican Council, Sacrosanctam Concilium, promulgated by Pope Blessed Paul VI states:
The Church acknowledges Gregorian chant as specially suited to the Roman liturgy: therefore, other things being equal, it should be given pride of place in liturgical services. SC 116
The development of Gregorian chant spans millennia and has been the sacred song of the Latin Mass since its earliest days. It is monophonic and unaccompanied.
Through Gregorian chant, the texts of the Mass are set to be sung by the priest and the faithful.
At Mass, there are two sets of chants, the Propers and the Ordinary.
The Propers are distinct texts for each Mass that change from Mass to Mass. They include the Introit, the Collect, the Epistle, the Gospel, the Offertory, the Secreto, the Preface, the Communion, and the Post-Communion. These parts of the Mass are chanted by the Priest or the Schola Cantorum.
The Ordinary are the parts of the Mass sung by the Faithful: the Kyrie, the Gloria, the Credo, the Sanctus, and the Agnus Dei. The different Ordinary settings reflect the season of the church.