Om is a very simple sound with a complex meaning. Om is the whole universe coalesced into a single word, representing the union of mind, body, and spirit that is at the heart of yoga. Often chanted three times at the start and finish of a yoga session, the sound of om is actually three syllables - a, u, and m.
Om is an ancient mantra that is used in Hinduism and Buddhism, among other faiths. In the Hindu tradition, the sound om is said to contain the entire universe. It is the first sound from the beginning of time, as well encompassing the present and the future. As such, its importance and power are difficult to overstate. It is also a seed syllable, used as a building block for other mantras. In the chakra system, it is connected to ajna chakra, the third eye, which represents intuition and self knowledge.