A draconarius is a person who rode on a dragon in ancient times. Kind of like how you ride a bike or a horse, but dragons are much bigger and have wings to fly!
Back in the day, people thought that dragons were real and sometimes they would use them in battles to scare away their enemies. A draconarius would sit on the back of the dragon and control it with special commands or signals. The dragon would flap its wings, soar through the sky, and breathe fire to protect its rider.
However, we now know that dragons are just mythical creatures and they do not really exist. But the idea of the draconarius is still cool to imagine!