The syllable OM is often chanted in the beginning and/or end of class. It has different meanings according to different schools, my school believes it has no meaning only vibrations. Below you see the different explanation for the syllable.
If you familiarize yourself with chanting, I am no expert, you´ll find that OM does vibrate all your four upper chakras, the Anatha, Vishuda, Ajna and Sahasrara; 4, 5, 6 & 7. Try it out for yourself and figure out how it is for you.
It is a cleansing mantra and relieves the yogi from all sin, Upanishads.
If you do nothing else but OM a few minutes a day you are on the yogic path.