The king was considered so important that people didn’t refer to him directly. They spoke of the “Palace” or “per-aa” instead. This is the origin of the title “pharaoh”. Kings had two different names: their “Son of Re” name, received at birth, and their nsw-bity name, received when they were crowned. Nsw-bity means “King of Upper and Lower Egypt”. We usually refer to kings by their Son of Re name.

Source: The Usbourne Encyclopedia of Ancient Egypt