Q. 1. During the Holy Mass, there is a part called the "Secret". What is the purpose of the "Secret"?

A. 1. According to Wikipedia, the free online encyclopedia which nicely explains the purpose of the "Secret",

"The Secret (Latin: Secreta, oratio secreta) is a prayer said in a low voice by the priest or Bishop during the Catholic liturgy, which includes the Holy Mass."

"In the Roman Catholic Rite the Secreta is said by the priest at the end of the Offertory in the Mass. It is the original and for a long time was the only offertory prayer. 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 the "Secreta") did not begin earlier than the sixth or seventh century. The present offertory prayers are late additions, not made in Rome until the fourteenth century. Before that the offertory act was made in silence, and the corresponding prayer that followed it was the Secret. Since it is said silently the Secret is not introduced by the invitation to the people: 'Oremus.'"

"The Secret is part of the Accentus of the Mass, changing for each feast or occasion, and is built up in the same way as the Collect. The Secret too alludes to the saint or occasion of the day. But it keeps its special character inasmuch as it nearly always (always in the case of the old ones) asks God to receive the eucharist and sanctify it. All this is found exactly as now in the earliest Secrets we know, those of the Leonine Sacramentary. Already there the Collect, Secret, Postcommunion, and 'Oratio ad populum' form a connected and homogeneous group of prayers. So the multiplication of Collects in one Mass entailed a corresponding multiplication of Secrets. For every Collect the corresponding Secret is said."

[Source: https://en.wikipedia.org/wiki/Secret_(liturgy) ]