A chain is put around the Drive Pulley Sprocket and the motor sprocket.
The chain moves when the motor is started. The chain turns the drive pulley.
The motor is connected to the reducer with a V-Belt (like the fan belt in your car) or a "C" Face Coupling.
The reducer is connected to the drive pulley with a chain . . . Now the Drive Pulley turns slower.
All those parts (the motor, the speed reducer, and the drive pulley) are collectively called the conveyor drive.
Drives must take up as little space as possible, so the motor and reducer are put under and within the conveyor bed.
The tail pulley is located at the tail end of the conveyor and it turns freely. Now a "belt" can be put around the pulleys.
The Drive Pulley turns and moves the belt around (and around, and around). But it is dangerous to have the belt hang down under the conveyor bed. So small rollers are put into the Conveyor Bed to hold up the belt.
These turn freely -- and just idle. They are called "Return Idlers".