Now therefore it would seem to follow, that both he who knows this (the true meaning of the syllable Om), and he who does not, perform the same sacrifice. But this is not so, for knowledge and ignorance are different. The sacrifice which a man performs with knowledge, faith, and the Upanishad is more powerful. This is the full account of the syllable Om.

In plain words: Someone might think that a person who understands Om and one who doesn't are doing the same ritual. But they are not, because knowing and not knowing are different. A ritual done with understanding and faith is more powerful.