“When you sing you pray twice”
St Augustine
The word mantra is a sanskrit word that is derived from the word man (meaning mind) and trai (meaning to protect or instrument or tool). Therefore mantras are tools  that protect or free the mind.
The first fundamental use of mantra while meditating, is that it helps to focus the mind. When we sit down to meditate our mind wanders and gets lost in thought. Paying attention is difficult. Chanting a mantra helps to anchor the mind in concentration. Of course background mind chatter may still exist even while reciting a mantra but it is significantly reduced and it allows the mind a point of focus, something to come back to again and again. 
A second aspect of  mantra  relates to sound and vibration. Sound is vibration. All the cells in your body are vibrating. Everything in the universe is vibrating. Your thoughts and feelings are actually vibrations. They vibrate a certain frequency in your mind and body.
Mantras are repetitive sounds that penetrate your being and adjust your thoughts and feelings by adjusting the frequency at which they vibrate. Thus  when repeated for a period of time they create profound changes in your body and psyche. In Kundalini yoga most mantras are recited in Gurmukhi (some are in English).The words guru-mukh literally mean “from the mouth of the guru.” Although mantras can be chanted in any language, certain languages are designed to have more of an impact in creating targeted psychological and spiritual effects. Gurmukhi is such a language. It was designed to encapsulate the consciousness of the first Sikh gurus. We know how deeply sound can affect us by noticing how a beautiful piece of music makes us feel as opposed to hearing a shouting angry voice. Reciting a mantra during meditation is a powerful refinement of this type of phenomena.
Another aspect to consider when chanting mantras is that the  act of your tongue pressing against the palate of the mouth stimulates the meridians or energy channels in the roof of your mouth and in turn stimulates hypothalamus, thalamus, and pituitary. When you recite a mantra, you hit the palate with your tongue in a specific sequence over and over again. Different mantras will have a different sequence of interacting with the meridians and thus in conjunction with the different sound vibration produce a different result in your thoughts, feelings and consciousness. In the palate, there are 84 meridian points. Some people use the analogy of  an ATM. The mantra helps you touch the roof of the mouth with your tongue in a specific order (punching in your pin number) that stimulates the meridians that in turn unlock the pituitary gland and allow it to secrete in a specific way (cash out). Interestingly, the pituitary gland is formed in the fetus from cells in the roof of the mouth that rise into the brain. This is why pressing the tongue against the roof of the mouth, a common practice in Kundalini, stimulates the gland so effectively. 
Here are some Kundalini mantras that you will hear in class.
Ong Namo Guru Dev Namo
This mantra—used to “Tune In”—opens every Kundalini yoga class. The words mean “I honor (or bow to) the Infinite Wisdom, I honor the teacher within.” The mantra is chanted three times.
Aad Guray Nameh
“Aad Guray Nameh, Jugaad Guray Nameh, Sat Guray Nameh, Siri Guru Dav-vay Nameh.”
This mantra is for guidance and protection and is often chanted three times following the opening mantra (Ong Namo Guru Dev Namo).
Sat Nam
 “I am truth,” “Truth is my identity” or “Truth is my essence.” It’s chanted one to three times, at the end of each class—much like Om is used in many other yoga traditions