The major importance of Operation Sea Lion is that it was never carried out. This was a plan that never actually happened.
Operation Sea Lion was the name given to the planned invasion of Britain by the Germans early in World War II. The Germans, having easily swept across Western Europe, were planning to invade Britain as well. The operation was not given a specific planned date, but it is widely believed that it would have happened in the fall of 1940.
This operation was never carried out because the German air force was never able to achieve air or naval superiority over the English Channel. Without this, the operation would have been too dangerous as British airplanes and ships could have devastated the flotilla bringing the invading soldiers.