A great circle is the path that a light or radio beam would follow.
Not quite so sure about that as, with the exception of lasers, electromagnetic radiation likes to propagate on a spherical wavefront.
A great circle is the shortest distance between two points along the surface of the Earth ignoring any mountain ranges. A great circle taken all the way round between said two points effectively divides the Earth into two equal hemispheres -- the Equator being one case; others being any line of longitude paired with its opposite number (+ or - 180 degrees).