In Transcendental Meditation we learn a mantra, or sound, as a vehicle, to serve as a vehicle to allow the attention of the mind to take this inward direction. And a mantra is defined as just a sound that is known to be positive and life supporting, and has no particular meaning. So we use this mantra as a vehicle to experience quieter levels of thought and quieter levels of thoughts, and then to transcend thought, to go beyond thought, to experience the source of thought, that pure consciousness, that unbounded level of the mind, that source of creativity and energy and intelligence within. And when we learn Transcendental Meditation, we learn from a teacher of TM a particular sound and then how to use it properly..
As found on Youtube