The breath (apana or vata) is the air that moves in and out of the body with the rise and fall of the diaphragm. Like most people, the ancient Indians associated life with respiration and in fact one of the Pali words for animal life is apana, literally ‘breathing things.’
The first Precept actually says: ‘I take the precept not to harm breathing things’ (panati pata…), meaning that bacteria, sponges, plants, etc. are not included in the Precept. Because of the connection between life and respiration, the Indians saw the breath as having some mystical significance. Ascetics also noticed that holding the breath, or breathing rapidly for extended periods, would cause hallucinations, which they interpreted as an exalted state. Consequently, many of the types of meditation popular during the Buddha’s the time focused on the breath. Before his enlightenment, one of the practices the Buddha experimented with was ‘breath retention meditation’ (appanakam jhanam), which he finally gave up as making his body overwrought and agitated and being painful (M.I,243-4).
Although the Buddha taught a meditation based on the movement of the breath (anapana sati), he did not do so because he believed it has any mystical power or significance. So why the breath? I suspect that there are three reasons for this. The first is purely practical. (1) The breath is a convenient object to focus attention on and it is available to everyone. (2) the breath’s gentle in and out movement has a natural ability to calm the mind. (3) Focusing on the breath can be the first step in drawing attention away from external distractions to the mind. Many mental states cause some change in the breathing. When we are calm our breath is long, slow and gentle, and when we are excited it becomes short and fast. We hold our breath in expectation, huff with annoyance, sigh with sadness, get exasperated and breath free with relief. Watching the movement of our breath naturally leads to becoming aware of the movement of our mind.