Reichsmarschall is a fancy German word that means "Marshal of the Reich." A Reich is a word that the Germans used to describe their country during World War II. Think of it like a big kingdom where the leader is in charge of everything.
Now, a marshal is someone who is really important in the army. They are like the leader of all the soldiers and they make really important decisions.
During World War II, there was a man named Hermann Göring who was given the title of Reichsmarschall by his boss, a guy named Adolf Hitler. This title meant that Göring was the most important person in the German army, and he was in charge of everything that had to do with the military.
So, imagine you are playing with your toy army men and you have one special guy that is the leader of all the other soldiers. That is kind of like what a Reichsmarschall was - the most important, top leader of the German army.