the numbers refer to the final gear ratio at the rear axle from the engine to the tires. your first step down is the transmission. the number is the ratio between the driveshaft rotational speed and the axles/tires rotational speed. 3.73 means that the driveshaft has to make 3.73 revolutions/rotations for one complete revolution/rotation of the axles/tires.
the higher the gear, the higher your engine will have to rev for every speed and the more torque you will get at the wheels. one thing to note though is that higher gears do not change the output torque of the engine, they just multiply it by a higher number so you get more to the wheels.