Nēna, or Nān is the third of the Three Worlds/Universes, which is the physical universe. Everything that you can see and feel and touch and experience, as well the near worlds which most people can’t currently sense (like the places where ghosts reside, the fae realms, etc.), which all combined make up the universe that we as incarnate being experience, is Nān. The letter is called either Nēna or Nān, when talking specifically about the letter in a linguistic sense it’s called Nēna, when talking about what the letter means, i.e. the realm of influence that this letter covers, you usually say Nān.
The original hieroglyph came from the image of two pillar gods standing between a ceiling and a floor, symbolising just the one world.
When reading this symbol without any diacritic marks the pronunciation is “na”.