What is the explanation of the hadith: "He would seek forgiveness for the first row three times, and for the second row once," and is it said by the Imam only, and is it said silently or aloud, and when is it said?

The Prophet's seeking of forgiveness (istighfar) for the first row three times and for the second row once, and his supplication (du'a) for those who shaved their heads (muhalliqeen) three times and for those who only shortened their hair (muqassireen) once, is specific to him (may Allah's peace and blessings be upon him). This was to demonstrate the virtue of the advanced rows and the superiority of shaving over shortening the hair. It is not a Sunnah for the Imam to do so. This supplication encompasses the entire Ummah and is in accordance with Allah's and His angels' prayers upon the advanced rows.