Reich means state, kingdom or empire.
The First Reich was the Holy Roman Empire, which lasted from 800 to 1806, when it collapsed under pressure from Napoleon's France.
The Second Reich describes the period which began with Germany's first unification as a modern state under Kaiser Wilhelm and his chancellor, Otto von Bismarck. It lasted from 1871 to 1918.
Hitler termed Nazi Germany (1933-1945) the Third Reich to capitalize on the glory of the previous Reichs.