"Serve, Love, Give, Purify, Medidate, Realise. Be good, do good, be kind, be compassionate."
At the end of yoga class, sometimes even before, your teacher may lead everyone in chanting OM (aum).
What is Om?
Om is called the "Pranava," a Sanskrit word meaining life force (prana) and life-giver.
"Serve, Love, Give, Purify, Meditate, Realise. Be good, do good, be kind, be compassionate.
I encourage my students to participate in chanting Om, although its not required. Chanting OM is perfectly harmless and can be done regardless of your spiritual beliefs. It’s actually really satisfying sound to just let rip!
You should try it...don't be shy...one of the most important things that yoga teaches us is not to place judgment on yourself or others.
Why do we "Om?"
You can meditate with the mantra Om, this is called “Om yoga”. Try it...repeat “om” in unison with your breath. The purpose of this is to become free from suffering and limitation. Om yoga is discussed in many sacred yoga and Hindu texts, such as the Upanishads, the Bhagavad Gita, and Patanjali’s Yoga Sutras. 
Some people find that the chanting of helps their breathing become more regular and slower. aiding in mental focus and relaxation, hence why yogis like to start or end their yoga practice by chanting Om.
“Om,”A Little Word but Powerful Meaning
There is a ton of information on the Web about the meaning behind Om. You may even notice is sounds similar to "home" and "amen" when you chant those words, comforting your the mind and soul.
Here are some of the writings about Om from ancient writings that helped form yoga traditions:
-
Om is the root of all sounds. Every other sound is contained in That, and It is used to take one beyond all sound. - Sri Anandamovi Ma
- The Self is of the nature of the Syllable Om…Meditate on Om as the Self - Mandukya Upanishad 1.8.12, 2.2.3
- God’s voice is Om –Yoga Sutras of Patanjali 1:27