He asked: 'And who is Indra, and who is Pragapati?' Yagnavalkya replied: 'Indra is thunder, Pragapati is the sacrifice.' He asked: 'And what is the thunder?' Yagnavalkya replied: 'The thunderbolt.' He asked: 'And what is the sacrifice?' Yagnavalkya replied: 'The (sacrificial) animals.'

In plain words: Yagnavalkya says Indra, king of the gods, is thunder, which means the thunderbolt. Pragapati, the lord of creatures, is the sacrifice, which means the animals offered in it.