A contrast adjustment with a particular shape. Everything below edge0 is crushed to black, everything above edge1 is blown to white, and the range between is stretched across the full scale with a gentle S-curve.
The S is the point. A straight remap would clip harshly at both ends; this eases in and out, so the shadows and highlights approach their limits gradually rather than hitting a wall. It is the same curve used for soft edges throughout the book.
Each channel is remapped separately against the same pair of edges, which means a colour cast in the picture gets exaggerated rather than corrected. If you want even-handed contrast, use adjust() instead.
Setting edge0 above edge1 inverts the result, which is a quick way to get a high-contrast negative.