What is meant by "the first row" mentioned in the Hadith: "If they knew what was in the first row, they would come to it even if crawling"?

The intended meaning of "the first row" in the Hadith: "If people knew what was in the call to prayer and the first row..." is the row immediately behind the Imam. Al-Hafiz Ibn Hajar clarified this in Al-Fath, and Al-Nawawi mentioned that this is the correct and chosen opinion, whereas the other two opinions (the first complete row after the Imam, not one with gaps, and whoever came early to prayer even if they prayed in the back rows) are clearly incorrect.