Basically, a spinning body exerts a "force" (like centrifugal force, the Coriolois force is not truly a force, merely a by-product of intertia) that causes the paths of objects on the spinning body to be deflected. In ballistics, to aim a projectile at a target a few miles or more away, you must adjust for this effect- i.e., not aim exactly at your target, but shift your aim in one direction to correct the effect. The coriolis effect is also responsible for the direction in which tornados spin.