What does Khṣaṣ al-bab mean?

إِنَّ ٱللَّهَ وَمَلَٰٓئِكَتَهُۥ يُصَلُّونَ عَلَى ٱلنَّبِىِّ ۚ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ صَلُّوا۟ عَلَيْهِ وَسَلِّمُوا۟ تَسْلِيمًا

God and His angels shower their blessings on the Prophet. O believers, you should also send your blessings on him, and salute him with a worthy greeting.

Al-Ahzab 33:56