Flieger is the German word for aviator or airplane. A flieger watch is just another way of saying “pilot watch”. Originally, flieger watches were used by the German Luftwaffe. Today, flieger watches tend to be aviation inspired, legible, and often minimalist in overall design.
The flieger watch example above is an Elysee Graf Berghe von Trips.
Above is a Marc and Sons flieger watch.
The watch pictured above is a DELTAt SoRa WWII fleiger watch.