What is meant by the statement, "Performing congregational prayer in the mosque is Sunnah (recommended), not obligatory according to the soundest opinion; rather, if performed elsewhere, it suffices, though it is preferable to perform it in the mosque"?

The meaning of "Performing congregational prayer in the mosque is a Sunnah, not an obligation according to the sounder opinion. In fact, if it is performed outside the mosque, it is still valid" is that the obligation is to pray in congregation, and the mosque is not specifically required for this. So, if one prays in congregation at home, at work, or in any other place, the obligation has been fulfilled. However, it is preferable for the prayer to be performed in the mosque. Scholars have differed regarding the ruling on congregational prayer, specifically whether it is an individual obligation (wajib aini) in the mosque, a collective obligation (wajib kifayi) in the mosque, an individual obligation without specifying the mosque, or merely a recommended Sunnah.