It is a tool and a technique that aids sight-singing, dictation, understanding of intervals, chords, harmony, scales. It develops inner hearing and harmonic hearing, and actually helps you to perceive music in quite a different way. Difficult to describe in words.
I need to learn this so badly! I think it is at the heart of why, despite playing the piano for decades, I don't feel like a musician and can't sight sing or sight read very well. I have been wanting to go on a Kodaly course since finding out about them from Cyrilla a few years back but sadly, due to family commitments, I just can't at the moment. Do any online courses exist?