Reich, is German for empire (essentially) and meant the complete unification of Germans under a single leader.
The first Reich was the Holy Roman Empire (800-1806), the second was the Imperial German Empire (1871-1918).
Hitler called his state the third reich, to gain legitimacy from these two empires, and Germany’s history, and to symbolise that he, was the Fuhrer was the leader of all German peoples.
It should be noted he called his system, people government “The Third Reich”, and The land it was on Germany, much like you’d call the USA, The United States, and the land it is on America.
Hello, I answered this question previously here. I hope it's useful.