Then the Asuras said to him: 'Tell us something, Sir.' He told them the same syllable Da. Then he said: 'Did you understand?' They said: 'We did understand. You told us, "Dayadham," Be merciful.' 'Yes,' he said, 'you have understood.' The divine voice of thunder repeats the same, Da Da Da, that is, Be subdued, Give, Be merciful. Therefore let that triad be taught, Subduing, Giving, and Mercy.

In plain words: Then the demons asked, and Prajapati again said 'Da.' They understood it as 'Dayadhvam,' meaning 'be merciful,' and he agreed. Thunder still repeats 'Da Da Da' to remind everyone: control yourself, give, and be kind.