The surah describes those who sit and listen to Muhammad but leave his presence asking mockingly, "What did he just say?"
Surah 47:16:
And among them are those who listen to you, until when they depart from you they say to those who were given knowledge, 'What has he said just now?' Those are the ones of whom Allah has sealed their hearts and who have followed their desires.
Verse 24 adds:
Surah 47:24:
"Then do they not reflect upon the Qur'an, or are there locks upon their hearts?"
The Quran attributes the rejection of Muhammad strictly to psychological blindness or arrogance ("locks upon their hearts"). This shows there was contemporary Arabic speakers who sat directly in Muhammad's presence but walked away unable to comprehend or extract meaningful substance from his discourse.
A Christian can flip this: "We reject the Quran not because of 'locks on our hearts,' but because we do reflect on it and find that it violates the criteria of the previous scriptures. We are obeying Surah 46:4 by checking the prior books, and finding that Surah 47's theology of salvation and warfare completely contradicts them."