Let a man meditate on the syllable Om, called the udgîtha; for the udgîtha (a portion of the Sâma-veda) is sung, beginning with Om. The full account, however, of Om is this:—

In plain words: A person should meditate on the sacred syllable Om. It is also called the udgitha, a chant from the Sama Veda (a book of sacred hymns), because that chant begins with Om. The full explanation of Om follows.