The word mantra comes from a very beautiful experience; that of the sound waves moving the mind. Tra comes from tarang, which is the wave of sound. Man is the mind. When we chant sacred mantras, we are creating a sound wave that the mind feels safe to embark upon. This sound wave takes us out of our limited individual mind into our Infinite universal mind.
Guru Singh explains the mantra Ong Namo Guru Dev Namo.