He desired that this body should be fit for sacrifice (medhya), and that he should be embodied by it. Then he became a horse (asva), because it swelled (asvat), and was fit for sacrifice (medhya); and this is why the horse-sacrifice is called Asva-medha. Verily he who knows him thus, knows the Asvamedha. Then, letting the horse free, he thought, and at the end of a year he offered it up for himself, while he gave up the (other) animals to the deities. Therefore the sacrificers offered up the purified horse belonging to Pragapati, (as dedicated) to all the deities. Verily the shining sun is the Asvamedha-sacrifice, and his body is the year; Agni is the sacrificial fire (arka), and these worlds are his bodies. These two are the sacrificial fire and the Asvamedha-sacrifice, and they are again one deity, viz. Death. He (who knows this) overcomes another death, death does not reach him, death is his Self, he becomes one of those deities.

In plain words: Prajapati, the creator (here also seen as Death), wanted his swollen body to be fit for sacrifice, so it became a horse. That is how the great horse sacrifice, the Ashvamedha, got its name. The passage then says the sun is really this sacrifice and fire is the sacred fire, and both are one god, Death. Whoever understands this is no longer touched by death, because death has become his own Self.