It is said in a low voice merely because it was said at the same time the choir sang the Offertory, and it has inherited the special name of Secret as being the only prayer said in that way at the beginning.
The silent recital of the Canon (which is sometimes called "Secreta") did not begin earlier than the sixth or seventh century.
Already there the Collect, Secret, Postcommunion, and "Oratio ad populum" form a connected and homogeneous group of prayers.
The name "Secreta" is used in the Gelasian Sacramentary; in the Gregorian book these prayers have the title "Super oblata."
In the Gallican Rite there was also a variable offertory prayer introduced by an invitation to the people; it had no special name.
All the Eastern rites have prayers, now said silently, after the Great Entrance, when the gifts are brought to the altar and offered to God, but they are invariable and none of them can be exactly compared to the Roman Secret.
The last clause of the last Secret: "Per omnia sæcula sæculorum" is said or sung aloud, forming the ekphonesis before the Preface.
In the ordinary form of contemporary Catholic usage, the secret corresponds to the Prayer over the Offerings and is sung or recited audibly throughout by the celebrant.
Textually, the secret prayers are obviously intended to be said silently, often professing personal unworthiness on the part of the priest, and—though they are often written in the plural—they often contain references to the laity as distinct from the speaker(s), who are the clergy.