Rēda is the first of the Three Certainties of Life, which is motion/breath/growth. When something is alive, it usually moves or breathes in some way and that’s the essence of this symbol.
The original hieroglyph was either a river (which is always in motion) or a serpent/dragon. So the symbol means the very essence of what life is, which is movement and growth and breath. For example the name “Rana”, if translated into English, could be translated into “Gypsy”, as in someone who is always moving around.
The symbol without any diacritic marks is pronounced as “ra”, and technically with a tap or rolled “r” sound.