The meaning of Om:

Om (AUM) is a mystic syllable, considered the most sacred mantra in Hinduism and Tibetan Buddhism. I appears at the beginning and end of most Sanskrit recitations, prayers, and texts. It is a sound that designates or points to that which cannot be uttered in speech but which is the Source of us all and everything.
In another meaning of the syllable AUM (pronounced Om), "A" represents the waking state, "U" represent the dream state, "M" represents the deep sleep state, and silence represents the "4th" state called "Turiya" in Sanskrit, which is the transcendental or spiritually awakened state.

What is pranayama?

Pranayama is the practice of breath regulation. It is a major component of yoga, In Sanskrit "prana" means breath and "yama" means control. This practice can help to calm the mind and body, facilitate meditation, and promote general well-being.

Simple instructions:

Sit in a comfortable position on a chair with your feet flat on the ground or cross-legged with your spine erect.
Relax and play-back the music. (it's best to listen through earbuds or headphones if possible).
Just follow and inhale along with the recording and slowly exhale and chant "Om" along with the recording. (if the pace is too slow for you, you can inhale and exhale at a faster pace).
While exhaling "Om" (AUM), gently bring to mind each letter of the syllable as you chant it (see "The meaning of Om" above), and then be aware of the pause (silence) before the next inhale, which represents the spiritually awakened or transcendental state.
It can be helpful to keep a journal of your experiences while practicing this pranayama to see what progress you may make.
The recording is 10 minutes long, so just repeat if you wish to practice longer. It is suggested that you don't practice this for more than 30 minutes in one sitting.
Like any other practice, the results will be accumulative, so it is suggested that practice this consistently for a time (perhaps 30 days) before evaluating it's effectiveness.

Chanting "Om" can help to restore your mind, body, and spirit balance and has many other benefits. You can research further benefits of chanting "Om" on the internet.

One word of caution:
If you have any pre-existing health conditions, It may be wise to consult your doctor first before proceeding. In general, make sure that you are healthy enough for this type of practice