'Breath is Brahman, Ka (pleasure) is Brahman, Kha (ether) is Brahman.' He said: 'I understand that breath is Brahman, but I do not understand Ka or Kha.' They said: 'What is Ka is Kha, what is Kha is Ka.' They therefore taught him Brahman as breath, and as the ether (in the heart).

In plain words: The fires teach that Brahman is breath, joy (Ka) and open space (Kha). Upakosala understands breath but not the other two, so they explain that joy and space are one, teaching him Brahman as breath and as the space in the heart.