There are three mātrās ("letters", syllabic instants in prosody) in the word aum : ‘a’, ‘u’ and ‘m’. The ‘a’ stands for the state of wakefulness, where we experience externally through our mind and sense organs. The ‘u’ stands for the dream state, in which inward experiences are available. In the state of deep sleep, represented by the sound ‘m’, there is no desire and consciousness is gathered in upon itself.
But there is a fourth, transcendent state, that of one "who is neither inwardly nor outwardly aware, nor both inward and outward, nor with consciousness infolded on itself.... who is unseen and ineffable, ungraspable, featureless, unthinkable and unnameable" The fourth state (turīya avasthā) corresponds to silence as the other three correspond to AUM. It is the substratum of the other three states.
Hindus begin their day or any work or a journey by uttering Om. The sacred symbol is often found at the head of letters, at the beginning of examination papers and so on. Many Hindus, as an expression of spiritual perfection, wear the sign of Om as a pendant. In yoga, it is used as a symbol marking the beginning of and end of your routine.