The speech in this verse is about the hypocrites who used to dwell in the outskirts around al-Medina, called the Bedouins.

(وَجَاءَ الْمُعَذِّرُونَ مِنَ الْأَعْرَابِ لِيُؤْذَنَ لَهُمْ) (Some bedouin Arabs came with excuses, seeking exemption [from war].): It refers to those who make excuses for themselves because they want to escape fighting by presenting fabricated excuses.

وَقَعَدَ الَّذِينَ كَذَبُوا اللَّهَ وَرَسُولَهُ ۚ  (And those who were untrue to Allah and His Messenger remained behind [with no excuse].): They were left behind because of the excuses that they had brought forth and their request to be with the weak and helpless. They sought permission, and the noble Messenger granted it to them.