"As the crow flies" is a colloquial term used to describe the most direct route between two points on the Earth. It is most often used to differentiate with a less direct route, such as a road or railroad, and is generally the same as the great circle route.
An example would be the distance between Key West and Pensacola, at the two opposite ends of Florida. Driving, the distance is approximately 792 miles or 1275km, but directly across the Gulf of Mexico it is only around 500 miles or 800km as the crow flies.
