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.
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, Agna 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.