The Coriolis effect is a perceptual cause of apparent influence on a long range projectile's flight attributable to a continual change in frame of reference as it arcs high above a round and spinning planet. It is often falsely thought to be a physical force, but this is not the case.
By World War II, long range naval gunnery had evolved to the point that the Coriolis effect was treated as part of the technological synthesis of a gunnery solution in battle. In World War I, however, it was largely left untreated — and for sound reason.
Nature of the Effect
The Coriolis effect is caused by changes in the distance between a shell in flight across latitudes and the axis of the earth's rotation. If we consider the most extreme illustration of this, a shell fired from the North Pole along the Prime Meridian to arrive after a 1 hour time-of-flight at the equator, it is easier to visualise. In this simplified case, the shell watches the impact zone spin away from it for a full hour. The gunners, spinning with their mounting, will see the shell appear to wander to the right 15 degrees (360/24) from what they felt was its point of aim, as though an invisible hand had pushed it. If they had fired northward, the direction of apparent error in deflection would have been to the left. More realistically moderate differences in firing and impact latitude and time-of-flight would have more moderate influence on the discrepancy in firing bearing and observed impact bearing. It is an issue, however modest, in extremely long range gunnery.
Urban Legends of Coriolis Effect
There is a nearly inextinguishable urban legend that the Royal Navy's shooting at the Battle of the Falkland Islands was atrocious owing to their equipment applying corrections for Coriolis effect in the wrong direction, as the action was in the southern hemisphere rather than the northern. But in truth, no aspect of the Royal Navy's gunnery equipment or procedures took Coriolis effect into consideration at this juncture, and this was not a terrible deficiency by any means. For, even if the old story were true, and the action took place on a nearly constant bearing (as it did), and at a range that changed only slowly (as it did), even a blatant mistreatment of Coriolis effect would therefore have been a constant error, and one unlikely to be large compared to other factors affecting the proper deflection to use (such as the zig-zagging of a fleeing enemy). This implies that the remedy for such a miscue would have been a spotting correction for deflection which, once made, would counteract the error for the remainder of the action!
While I think it likely that later systems of firing incorporated Coriolis corrections nicely, a system lacking such treatment which is designed primarily to bring fire upon a maneuvering enemy is not a sad system by any means. Taken in context, failure to treat errors attributable to the Coriolis effect are a constant source of error (in range and/or deflection) and they are modest in scale. The need to fire repeated salvos which for many reasons will require spotting to put them onto the target implies that a failure to handle Coriolis effect, or even a failure to handle it correctly, does not result in an inability to hit the target.
Long range gunnery against a fleeing target is inherently a crap shoot.
Wikipedia has a detailed article on the Coriolis effect.