This (heart) indeed is even that, it was indeed the true (Brahman). And whosoever knows this great glorious first-born as the true Brahman, he conquers these worlds, and conquered likewise may that (enemy) be! yes, whosoever knows this great glorious first-born as the true Brahman; for Brahman is the true.

In plain words: The heart spoken of before is the true Brahman, the ultimate reality behind everything. Whoever knows this great first-born reality as the truth wins these worlds and overcomes his enemies, because Brahman is truth itself.