There are various explanations of the meaning of Om or Aum. If someone tells you that "Om" is the sound of God or the Universe, she is simply imagining things. I have a different interpretation. "Om" is the sound of a thoughtless state of mind - a present that is free from all tensions of life - the way to touch the depths of our real selves in its purest form.
The State of Om
The Sanskrit word "Aham", which means "I am", is a reminder
of my existence in life. A normal human being leads a life full of thoughts,
never getting to know his real self. "Om" is a derivative of "Aham"
that rhymes easily in a person's conscious being. When a person chants "Om"
she exists in her purest form without the thoughts disturbing her during the
sacred moments of spiritual existence.
The State of Existence
"Existence" indicates one state - the present. A happening yesterday
is in the past that "existed". I have to imagine and build thoughts
of the scenes of yesterday which is bound by time. When I plan for tomorrow,
the future "will exist". It belongs to future and I have to build
thoughts around the plan. The future is also time-bound. So, both past and
future are bound by time and thought.
However, when I remind myself of my existence, with the sound of "Om", it can be only in the present - a state where thoughts don't get born. The present state is timeless, unlimited and with freedom from thoughts, it is free from stress. So, a person chanting Om is without stress, in other words, healthy, efficient and wise.
The State of Meditation
Yoga scriptures advise a meditator to empty the mind and keep it free from all
thoughts. If I realize that thoughts are time bound, it will be clear that I
have to keep in the present state of "Om", where thoughts cannot occur.
In my present state I can only find my real self and this is a state of meditation.
To me meditation in life can be of two kinds: One, leading your life or Karma while always remaining in the present state of being. This brings high efficiency - without complications the whole being is dedicated to Karma in the "present". The other is as we know, when you sit for meditation in the mornings or evenings, and immerse ourselves in our thoughtless selves. In both cases, the present state serves timeless wisdom on a silver platter and you are at your efficient best.
With inputs from the Guide

