You've probably joined along with the final "Om" chant in your practice, knowing a little about the tradition and meaning, but mostly just enjoying the calming resonance it creates. I think you'll enjoy chanting even more if you know more about "Om" or "Aum".
Om is always within you, even babies make the sound. It is even within the sounds you hear in the real world everyday: the hum of the air conditioner, static on the radio, bagpipes. The Om sound connects you with the divine or the universe. It opens and activates all the chakras, especially the crown chakra.
Three is a magic number when it comes to Om. In Sanskrit, Om is spelled with three letters: A, U, and M and spoken with three parts of your mouth. The "ah" starts in the back of the mouth, then about a third of the way through the sound the "oh" begins to form in the middle. Finally, the last of Om ends with the front of the mouth, on the lips. When you finish each chant, be sure to relish in the quiet stillness it creates.
Additionally, each of the three parts of Om corresond to a part of life - "Ah" represents the creative and birth, "oh" corresponds to the sustaining and sameness of life, and the "m" is destruction and death. These also represent the past, present, and future - the moving from the awareness of the outer world to the inner world.
Further, the sounds of Om represent the four states of being: "ah" is our wakeful state, especially external awareness through the mind and senses. "Oh" is our dream state and our internal experiences. The "m" sound is representative of our deep sleep with no desires. And the silence between chants is our transcendent state, where we have the ability to merge with the universe (or at least just enjoy the quiet).
Traditionally, when chanting Om we use the Om mudra. This essentially is the "O.K" symbol - the tip of your index finger touching the thumb, creating a circle. This connects you (the index finger) with the universe (the thumb). There are many other mudras, each with their own meaning.