question #1 Sahih Muslim 200a, Book 1, Hadith 400 : What is the significance of the reserved prayer of the Prophet Muhammad for his Ummah?

Verily the Messenger of Allah (ﷺ) said: There is for every apostle a prayer with which he prays (to Allah) for his Ummah. I have reserved my prayer for the intercession of my Ummah on the Day of Resurrection.