What does a rear end do in a 18 wheeler?

Same thing it does in any other vehicle. "Rear end", "rears", etc. is commonly used in reference to the rear end gear ratio. If a truck has, for example, 3.58 rear end gears, it means the driveshaft turns 3.58 times for every one turn of the rear end gear. The rear end gears are what makes the tires of the drive axle turn, thus propelling the vehicle.