In plain words
The second ‘ja’ means Bhīma, the one born to the air: that is the meaning. Satisfied that there was nothing unknown to Kṛṣṇa, the all-powerful, the all-knowing, the all-pervading, Dharmaja described Him as one who has all knowledge: the knowledge of letters, of words and of their sense.
When Abhimanyu died, Arjuna in his fury addressed Kṛṣṇa as ‘destroyer of the line’. Since Arjuna was in a fury, Kṛṣṇa said nothing. With the thought, ‘However he reviles Me, it does not matter,’ He held on in patience. In childbirth Uttarā gave birth to a child that had died. Hearing this news, all the Pāṇḍavas were in grief. For Draupadī's sons, the Upapāṇḍavas, had died. Abhimanyu had died. Everyone had pinned their hopes on the child born to Uttarā as the one heir to the line. Unfortunately, to her too a dead child was born. Grieving that the Pāṇḍava line itself had died out, they sent Arjuna to bring Kṛṣṇa. When Kṛṣṇa came, Draupadī, Subhadrā and Uttarā were sitting shedding streams of tears. Draupadī went, laid the dead child in a winnowing basket, gave it to Kṛṣṇa and wept. Looking at the child, He said that it had Abhimanyu's looks in every feature. Then Draupadī said, ‘Yes, yes: like Abhimanyu, he is nothing but a corpse.’ Kṛṣṇa: ‘Chī! Chī! What words are these, sister!’ He said. The moment Kṛṣṇa said, ‘The child is very handsome and lively,’ all the Pāṇḍavas burst out sobbing. Kṛṣṇa turned the child this way and that, and as He breathed life into him, the child let out a loud cry. Hearing that sound, all the Pāṇḍavas were sunk in wonder. Arjuna fell at Kṛṣṇa's feet and said, ‘O Vāsudeva, who has kept alive the sprout of the Pāṇḍava line!’ Then Kṛṣṇa: ‘Brother-in-law! That day you called Me ‘destroyer of the line’, and today you say ‘You have given us the sprout of the line’. Is that right, or is this right?’ He said. Arjuna begged Him to forgive his ignorance. That is, in anger, in such a situation, a devotee reviles even God in many ways. From that day on they went on believing that Kṛṣṇa alone was their life. Because He examined the child and gave him life, they named him ‘Parīkṣit’.