1.28. The repetition of Om should be made with an understanding of its meaning.
This is a very deep subject and ultimately can only be understood intuitively. Coincidentally, I am writing this on the same day that Leonard Nemoy passed away. How better to explain the meaning of Om through the Vulcan philosophy of "infinite diversity in infinite combinations". Pretty mind blowing huh? That is what Om is, in a sense.
In Hindu mythology, the God brought forth the word in through the his voice of primordial vibration of Om. It's has it's parallel in the Bible "In the beginning was the Word, and the Word was with God, and the Word was God" It's the first verse in the Book of John. So, we Yogis tend to draw a parallel between the two for better or worse. So, with this, what was one broke into many, then into infinity, only to be brought back into the whole. That is how I understand it at least. By understanding a piece of the whole, we understand the nature of the whole itself, because the whole, by it's own nature, is indestructible and infinite.
So, if we chant Om enough, we will eventually, and intuitively understand it's meaning. Honestly, in practice, I have not had much luck doing this. In Yoga, there are thousands of Mantras you can use. I happened to have a racing mind, so longer mantras are better for me. When I have sat in meditation, in my the chanting has played out like this: Oooooommmm, Oooommmmm, Oooommmm, what's on the Walking Dead tonight?
Some practices in Kriya Yoga help one hear the vibration of Om by doing a practice called the Jyoti Mudra. It takes a long time to master, so if you Google it, keep that in that back of your head.