English Explanation of Mishnah Sanhedrin 4:4

And there were three rows of disciples of the Sages who sat before them, and each knew his proper place.
If they needed to appoint [another as a judge] they appointed him from the first row, and one from the second row came into the first row, and one from the third row came into the second row, and they chose another from the congregation and set him in the third row.
He did not sit in the place of the former, but he sat in the place that was proper for him.
Mishnah four discusses the disciples who sat in front of the Sages in the Sanhedrin and the procedure for a disciple’s appointment to the court.
This mishnah describe the seating arrangement of the disciples of the Sages (talmidei hachamim) who would sit and observe the proceedings of the Sanhedrin. There were three rows of official disciples, those waiting in the ranks to one day become judges. This was somewhat of an apprenticeship. If one of the judges had to leave or died, one of the disciples would take his place. The disciples themselves sat in rows according to their rank and when one would move up to be a judge, everyone behind him would move up in place. When the one from the second row moved up to the first, and the one in the third moved up to the second, and the one from the congregation moved up to the third, they would not sit in the beginning of the row but rather at the end of the row, which was their proper place.