A duke is a nobleman. In former times, certain important rulers had this title. Only the king and the emperor stood above the duke.
The Latin word "dux" means leader. In Ancient Rome, some leaders in the army were called dux. This title also existed in the Middle Ages. In the German language, it was changed to duke. In this word you can still recognise a little bit of the words "Heer" (army) and "ziehen" (to move) in the sense of going off.
Whoever was in charge of the army often became very powerful in other ways as well. The highest warriors were the kings. Dukes were less important and served their king. Just as with kings, duke was later mainly a title for a ruler. A grand duke was a title above the duke.
In Germany and Austria, the nobility was still allowed to determine some things until 1918. In Germany, for example, there was a Grand Duchy of Baden and a Duchy of Brunswick. In Austria, high nobles from the Habsburg family were allowed to call themselves archdukes.
Of all the duchies in Europe, only one remains: Luxembourg still has a grand duke. Moreover, elsewhere, duke has remained an important title. Some children and grandchildren of the English queen may call themselves duke or duke, in English "duke" and "duchess".