In Tibetan Buddhism, mantras are sacred syllables that evoke the energy of a buddha. We say them to wake that divine energy up in ourselves.  Mantras are intended to protect the mind from falling asleep to our true nature by reminding us that we have buddha nature. When we say them, we're calling on the energy of the buddha whose mantra we're saying (like Green Tara with OM TARE TUTTARE TURE SOHA, or Avalokiteshvara with OM MANI PADME HUM).  

In this audio from a talk I did on Insight Timer, I'll explain what mantras are and how to use them in your meditation practice, even if you're a beginner.  

You can find me on Insight Timer to participate in future live talks: http://insig.ht/clairevillarreal