In plain words
What we must learn from this is that whatever we give to God becomes inexhaustible.
Whatever God does, there is a reason for it
One Saṅkrānti day Kṛṣṇa, Satyabhāmā, Rukmiṇī and Draupadī were all eating sugarcane together. Just then, as Kṛṣṇa was cutting the cane, His little finger was cut. To bind it, Satyabhāmā and Rukmiṇī sent their maids to fetch cloth. But Draupadī at once tore her new sari and bound Kṛṣṇa's little finger. In return for that kindness, when they tried to strip Draupadī in the Kaurava court, Kṛṣṇa saved her by giving her saris without end. So whatever we offer to God, we have the right to receive it back. Every act of God has a reason.
Karṇa was under a curse. He made his guru Paraśurāma believe he was a brāhmaṇa and learned his skills from him. In the end it came out that he was a kṣatriya. Paraśurāma cursed Karṇa that the skills he had learned by deceiving him would be of no use. One day after the war was over, Kṛṣṇa told Arjuna to get down from the chariot. Arjuna, out of Ahaṅkāra, told Him to get down first; after they had argued back and forth like this, Arjuna at last got down from the chariot. The moment Kṛṣṇa stepped down from it, the chariot was burned to ashes. The reason: Karṇa's weapons would long ago have burned Arjuna's chariot to ashes. But Kṛṣṇa had been protecting it all along. Then Arjuna gave up his Ahaṅkāra and pleaded with Kṛṣṇa. Unable to withstand the force of the weapons Bhīṣma loosed, Arjuna called Kṛṣṇa to mind and prayed, ‘The Kṛṣṇa who created me must Himself protect me; the Kṛṣṇa who trained me must Himself be my shield.’ Seeing Bhīṣma's might, Arjuna grew weak.