AH represents the emptiness/voidness of anything. For example, the lines and curves that you are reading right now are empty of the sound or meaning that we have been educated to perceive or interpret as letters and words. Voidness is not mystical, exotic, or symbolic. Emptiness is stating that nothing inherently contains or is what an onlooker or experiencer is experiencing.
Examples abound that display this. Some people like poodles, some bull dogs. Some people enjoy spicy food, others prefer less kick. Some people enjoy jazz, others rap, others folk or classical. Some enjoy it all. These differences in taste or preference are possible because the food, the dogs, the music are not inherently as anyone perceives them, not even the creator of it such as an author, chef, or musician.
We meditate OM AH HUM OM adding AH to red at the throat center and to white OM at the crown center.