In the context of yoga practice, "om" is more of a sound than a word that has meaning. However, this doesn't mean that the om chant, which typically occurs at the beginning and/or end of a yoga session, is meaningless. Rather, this sound is a meditative chant.
The om chant is intended to help a yoga practitioner feel connected to the vibrations and movement of the world around them, extending all the way out from a yoga studio or practice room through the earth and even out into the universe. The vibrational quality of the sound produced by the om chant is more important than any specific signifier.