This (the breath in the mouth) and that (the sun) are the same. This is hot and that is hot. This they call svara (sound), and that they call pratyâsvara (reflected sound). Therefore let a man meditate on the udgîtha (Om) as this and that (as breath and as sun).

In plain words: The breath in the mouth and the sun are really the same. Both are warm. The breath is called sound and the sun is called echoing sound. So one should meditate on Om as both the breath and the sun.