The Ineffable Name of G-d
During these prayers on this awesome day, the High Priest would utter the Ineffable Name of G-d known as the Tetragrammaton. In Hebrew, this is known as G-d's "proper name" (Shem HaMeforash) and denotes the Holy One as the ultimate source of all existence. This most holy name is ordinarily not pronounced as it is written, and is not used at all outside of the Holy Temple. Even in the Temple it is used infrequently. During the course of the services on the Day of Atonement, the High Priest will have occasion to utter it 10 times.
The People's Response
In this confession, the High Priest pronounces this name 3 times. When the congregation who are assembled in the court hear the holy name of G-d from the lips of the High Priest, they collectively respond "Blessed be the Name of His glorious kingdom, for ever and ever," and prostrate themselves on the ground.
This response is based on the verse in Moses' song, (Deut. 32:3), "When I call upon the name of the Lord, give greatness to our G-d." The sages explain this to mean that Moses told Israel, "Whenever I mention the Holy One's name, you should ascribe greatness to our G-d."