Mudir is a word in the Fremen language and means "governor".
The word was used for the chief riders of Sandworms and had the privilege to command the steersmen. A youth usually rides the worms only as a passenger or steersman. It was customary at age 12 to call his first maker. The naib of his sietch along with various other men and a Sayyadina. accompanied him to the sand. The naib spoke the words of the ritual developed over the centuries to the would-be sandrider; the other men loaned thumper and hooks (it was considered ill luck for a boy not yet a rider to own such things); the Sayyadina, aloof from the proceedings, observed so that the events of the day could be properly recorded. Most boys were successful once the ways of the worm were better known.
At his "Haiiiii-yoh!" the steersmen would mount the worm behind him, followed by the rest of the witnesses. Then, following his calls of "Ach" (left turn) or "Derch" (right turn), they would guide the monster as he wished it guided. Not even the Naib of the youth's sietch would counter his orders until the ride had run its course.
The young mudir was first on and last off (a position that could be dangerous if the worm was still fresh and prepared to turn when the annoying hooks were removed). During first rides, the worm was usually ridden nearly into exhaustion; it would be far more eager to escape and rest than to attack