Bhashya
कतम इन्द्रः कतमः प्रजापतिरिति, स्तनयित्नुरेवेन्द्रोयज्ञः प्रजापतिरिति, कतमः स्तनयित्नुरित्यशनिरिति ।अ-शनिः वज्रं वीर्यं बलम्, यत् प्राणिनः प्रमापयति, सइन्द्रः ; इन्द्रस्य हि तत् कर्म ।कतमो यज्ञ इति पशवइति-यज्ञस्य हि साधनानि पशवः ; यज्ञस्यारूपत्वात् पशु-साधनाश्रयत्वाच्च पशवो यज्ञ इत्युच्यते ॥
In plain words
‘Which is Indra, which is Prajāpati?’ ‘Thunder itself is Indra; sacrifice is Prajāpati.’ ‘And which is thunder?’ ‘The thunderbolt.’ The thunderbolt is that raw force, that strength, which strikes living beings dead — and that force is Indra, because killing is exactly what Indra does. ‘And which is sacrifice?’ ‘The animals.’ Because the animals are what a sacrifice is actually made of — sacrifice has no shape of its own, it exists only through them — the animals themselves come to be called ‘sacrifice.’
Close to the Sanskrit
'Which is Indra, which is Prajāpati?' — 'Thunder alone is Indra, sacrifice is Prajāpati.' 'Which is thunder?' — 'The thunderbolt.' The thunderbolt is the vajra-weapon, potency, strength — that which causes living beings to die; that is Indra. For that is Indra's function. 'Which is sacrifice?' — 'The sacrificial animals' — for the animals are the means of the sacrifice. Because the sacrifice has no form of its own, and because it depends on the animals as its means, the animals are called 'sacrifice'.