Ah, is the first letter of the alphabet and the one that many of the DY teachings suggest to focus on in beginning dream yoga exercises. It represents opening, I believe. The other letters have meanings as I recall but don’t know what they are. Ah is also the first sound and open-arms practice used in Eurythmy, the visual speech taught at Rudolf Steiner’s Waldorf Schools (Mystical Christian) and it too represents opening oneself up. Hope this helps.
Note: from Rinpoche, Tenzin Wangyal. The Tibetan Yogas of Dream and Sleep (p. 167). Shambhala. Kindle Edition.
Visualize a beautiful red lotus with four petals in the throat chakra. The throat chakra is at the base of the throat, closer to where the neck meets the shoulders than to the head. In the center of the four petals, facing forward, is an upright, luminous Tibetan A, clear and translucent, like crystal made of pure light. Just as a crystal laid on red cloth reflects the color and appears red, so does the A pick up the red of the petals and appear red. On each of the four petals is a syllable: RA to the front, LA to your left, SHA to the back, and SA to the right. As sleep comes, maintain a light, relaxed focus on the A.
This part of the practice is meant to bring the mind and prana into the central channel. The quality is peaceful, and as we merge with the deep red A we find peace within ourselves. The teaching says that focusing on this chakra produces gentle dreams. The example given is of a dream in which a dakini gently invites the dreamer to accompany her. She helps the dreamer onto a mystical bird (garuda) or a lion and leads him or her to a pure land, a beautiful, sacred place. But the dream need not be this specific. It may just involve a walk in a beautiful garden or in the mountains, guided by other people. The quality of dreams generated has less to do with particular images and more to do, at this point, with the feeling of peace.