We are in Maseches Yoma, chapter 1, mishnah 6. As we mentioned earlier, they did not want the Kohen Gadol to sleep on the night of Yom Kippur, lest he experience a seminal emission, become tamei, and be disqualified from performing the service. The mishnah therefore lists the activities that were carried out to keep him awake all through that night.
Expounding and reading before the Kohen Gadol:
"Im hayah chacham - doresh" - if the Kohen Gadol was a talmid chacham, he himself would expound and deliver words of Torah before those present.
"V'im lav - talmidei chachamim dorshin lefanav" - and if he was not a talmid chacham, Torah scholars and darshanim would expound before him on the verses.
"V'im ragil likros - korei" - if he was accustomed to reading the sacred writings, he would read on his own.
"V'im lav - korin lefanav" - and if he was not accustomed to reading, others would read before him.
And from what would they read before him?
The mishnah lists the books: "B'Iyov u'v'Ezra u'v'Divrei Hayamim" - books from Kesuvim whose content is interesting and engaging, and which have the power to draw the heart and keep the Kohen Gadol awake. The mishnah adds: "Zecharyah ben Kevutal omer: pe'amim harbeh karisi lefanav b'Daniel" - that is, many times he read before him from the book of Daniel.
Rabbi Unterman's observation:
The Kehati commentary brings an interesting observation in the name of Rabbi Unterman, who served as Chief Rabbi of Israel. Rabbi Unterman pointed out that the first seven chapters of the book of Daniel are not written in Hebrew but in Aramaic, which was the spoken and common language in the days of the Second Beis Hamikdash. In his view, Zecharyah ben Kevutal was hinting that many of the Kohanim Gedolim of that period were unlearned men, to the point that they could not even speak in Lashon Hakodesh, but only in the everyday language, Aramaic. Therefore it was necessary to read before them specifically from the book of Daniel, whose language is Aramaic, and not from the books written in Hebrew.
In summary: this mishnah lists the measures taken to keep the Kohen Gadol awake on the night of Yom Kippur - expositions that he himself delivered or that were delivered before him, and reading in Kesuvim either by him or by others, in the books of Iyov, Ezra and Divrei Hayamim, and also in Daniel, as Zecharyah ben Kevutal said. In the words of Zecharyah ben Kevutal, Rabbi Unterman found a hint to the spiritual state of the Kohanim Gedolim of the Second Temple period, who were not fluent in Lashon Hakodesh.