This is the name given to the part of the Eucharist that precedes the canon, also called anaphora. It literally means what is done before (praefacere) and is said as thanksgiving once the offerings have been received and the consecration or anamnesis is about to proceed.
That is why the soul of the preface is the feeling that is expressed when it is said “it is truly just and necessary that we give thanks to you, God, the omnipotent Father.” And it ends with the hymn of adoration and praise of “Holy, holy, holy.
Pedro Chico González, Dictionary of Catechesis and Religious Pedagogy, Editorial Bruño, Lima, Peru 2006
Source: Dictionary of Catechesis and Religious Pedagogy
(From Latin praefatĭo).
1m Prologue or introduction to a book.
2. m. Part of the mass that immediately precedes the canon.
Source: Catholic Encyclopedia