Among the statements that the Mishna (32a) says must be made in Hebrew are the blessings of the kohen gadol. The Mishna on our daf discusses these blessings that are made by the kohen gadol on Yom Kippur. After the kohen gadol finishes the special Temple service of the day, the hazan haKnesset ceremoniously hands a Sefer Torah to the rosh haKnesset, who hands it to the segan kohen gadol – the assistant high priest – who gives to it the kohen gadol. The kohen gadol then reads selections from the Torah and makes eight berakhot.
The Rosh points out that there is no clear source that these blessings must be made in Hebrew any more than other berakhot. He suggests that the sanctity of the day may be what encouraged the sages to establish that the service be done specifically in Hebrew.
In his commentary to the Mishna, the Rambam explains that the procedure of handing the Torah from one person to another is an attempt to honor the Temple and the proceedings by making it a ceremony that involves many people.
The rosh haKnesset was in charge of the ongoing activities of the beit haKnesset. In many communities this job was given to the individual who was the head of the community. In some synagogues we find a special seat reserved for the rosh haKnesset. During the Second Temple period there was a special synagogue that was situated on the Temple Mount where the people who came to the beit haMikdash would assemble for set prayers. It was in that synagogue that the sifrei Torah were kept.
The hazan haKnesset was more of a functionary – a shamash – who was responsible to make sure that things were kept in proper order. In some synagogues the hazan haKnesset also served as the teacher of young pupils, or was responsible for their learning. Since they occasionally also led the services, the term became used popularly to describe the individual who leads the services on a regular basis.