Consider the flying-wing shown. If , the compression waves from the apex will be felt within the angle m (mach angle). However, in the figure, every point on the leading edge of the wing is in the undisturbed supersonic flow, and cannot feel the compression and flow deceleration due to the other points. This is a wing with a "supersonic leading edge".
 

Now consider the arrow-wing shown below. Here, every point on the leading edge is within the region of disturbed (decelerated) flow caused by the apex. As a result, the Mach number at the leading edge is slightly smaller than the free-stream Mach number. This is a "subsonic leading edge". Note that the flow is still supersonic.