To put it simply, TM is a method of giving deep rest to the mind and the body.
Generally the best form of rest experienced by us is a sleep of average 7-8 hours. Normally we assume that we are ‘resting’ while we are sitting and next step towards better rest would be to lie down. People normally think that this is the maximum rest and of course physically that is the maximum rest that you can get. But then still we are breathing, so that means some activity is going on somewhere, and energy is still required for action going on primarily in brain.