I have noticed to those not in the know, they often wonder why Tibetan Buddhists normally do 3 prostrations. There are 2 types of prostrations; half and full prostrations.
This is half prostration.
This is full prostration.
Prostrations are done to the Triple Gems - the Buddha, the Dharma and the Sangha. The characters are OM AH HUM. Prostrations are often seen as a means of purifying one's body, speech and mind of karmic defilements, especially pride.
Why do we do prostrations?
We do not do them to endear ourselves to somebody else. We do not do them for the Buddha. Such concepts are completely wrong. The Buddha is not a god of this world. We bow down to purify all situations from the past where we did not respect others. Being interested in our own satisfaction and ourselves we did many negative actions.
Prostrations help us realize that there is something more meaningful than ourselves. In this way we purify the pride that we have accumulated through countless lifetimes thinking: "I am right," "I am better than others," or "I am the most important one." During countless lifetimes we have developed pride that is the cause of our actions and have accumulated the karma that is a source of our suffering and problems. The goal of prostrations is to purify this karma and to change our mind set. Prostrations help us rely on something more meaningful than our pride and ego clinging. In this way, through full confidence and devotion, we get rid of everything we have gathered because of pride.
When we do prostrations we act on the level of body, speech, and mind. The result of doing them is a very powerful and thorough purification. This practice dissolves all impurities, regardless of their kind, because they were all accumulated through our body, speech, and mind. Prostrations purify on all three levels. Through the physical aspect of prostrating we purify our body. We offer our body to the Three Jewels (Buddha, Dharma, Sangha) and to all sentient beings, wishing that all their wishes are fulfilled.
Below are 2 short clips which Venerable Thubten Chodron explains in detail why and how to do prostrations: