'A' represents the physical plane;
'U' represents the mental and astral planes, the world of spirits and all heavens;
'M' represents the deep sleep state and all that is unknown even in our wakeful state and all that is beyond the reach of the intellect.
Om represents ALL and is the basis of our life, thought
(from 'Bliss Divine' - Swami Sivananda).
To me, Om is the first sound, the 'big bang!' and
universal life-force energy. Everything has its own vibration and
these subtle energies impact on everything around us.
'tropy'- I took from 'entropy' which denotes a measure of the efficiency of a system (law of thermodynamics]; or a lack of pattern or organization. Again there is a relationship to this energy, vibration and also chaos. Everything we do, think, say and believe has a positive or negative effect on this universal energy (some call this karma).