It means that soldiers yield to the general's orders, just like grass moves with the wind. Military orders are like mountains, and every soldier must strictly abide by them.

Wei Yun shouted to the outside: "People come from high and low houses." Soldiers will let the grass move with the wind. People came down from the room in a panic. Crow Crow is half standing in the yard. (Feng Zhi's Four Chapters of Armed Forces Behind the Enemy)