Om/Aum is Dravidian in origin, having the meaning of “Yes.” It has the literal meaning of “It is” or “Will be”. It is the aorist future form of Agu “to become”.
Om seems similar to the christian term Amen which means roughly speaking “so be it”. The word Om is often written at the end of upanishads, upanishads are short lessons on hindu spirituality. They can be found here:
The purpose of Om is to bring one into the level of being and thus center oneself. Om really is what one makes of it. For some it is a mantra of centering and for some they feel as though to use this mantra helps them unite with the ocean of being that is the cosmos. The ocean of being known as Brahman, the cosmic intelligence that is the natural intelligence permeating all phenomena known as God.
Ones intent, what one focuses on with their mental concentration, is very important. That has an influence upon how one experiences a mantra. One understanding of a mantra affects ones experience of the mantra, people can use the same mantras for different purposes as was outlines with examples of two different uses for the Om mantra.
Utilizing the Om mantra in meditation is called pranava yoga. One can say Om aloud or one can say it silently through thinking it whilst in meditation.