A belt connects the motor to the drum. If that belt breaks or is too loose, the drum will not turn and the clothes will not dry properly. Inspect the belt for proper tension, breakage and excessive wear.
Before inspecting the belt, unplug the dryer or shut off the power at the fuse box or breaker panel to avoid an electrical shock hazard. Turn off the gas supply to your dryer.
Access to the motor and belt may be easiest from the front or from the rear depending on your model of dryer. If you are uncertain of the location of the motor, start by removing the lower front panel. It should lift up and off or it may be held in place with a few screws. If you cannot reach the motor from the front, then you will have to remove the rear access panel which is held in place by screws along the outer edges.
The belt loops over the drum, under the idler pulley and around the drive pulley on the motor. The idler pulley (or tensioner) is a wheel on a flexible bracket that keeps tension on the belt.
When reaching into the belt area be careful of sharp edges on the cabinet and interior supports. Reach in to the belt and give it tug. It should be taught and have only a little give. Also, inspect the belt for cracks or wear. If you see cracks or the reinforcing fibers show through, the belt should be replaced.