BG Chap 18 - Kṛṣṇa makes sure Arjuna understood His teachings

Kṛṣṇa asks Arjuna if Arjuna had heard what Kṛṣṇa had taught with an attentive mind. He asks this because He was willing to teach Arjuna again if Arjuna had not understood it. The Lord was acting as the spiritual master of Arjuna. Therefore it was His duty to inquire from Arjuna whether he understood the whole Bhagavad-Gītā in its proper perspective. If not, the Lord was ready to re-explain any point, or the whole Bhagavad-Gītā if so required. Actually, anyone who hears Bhagavad-Gītā from a bona fide spiritual master like Kṛṣṇa or His representative will find that all his ignorance is dispelled.