Makes a picture look crisper by exaggerating the difference between each pixel and its surroundings.
The method is older than computers. Compare a pixel with the average of its neighbours: on a flat area they are the same and nothing happens, but on an edge the pixel differs, and that difference gets added back on top, pushing it further from its neighbours than it was. Edges gain contrast, flat areas are untouched. The weights are arranged so they add up to exactly one, which means the picture's overall brightness comes out unchanged.
amount does something slightly surprising. Rather than sharpening harder, it moves the neighbours it compares against further away. So turning it up does not make fine detail more intense, it starts sharpening larger and larger features instead. If you want fine detail brought up hard, unsharpMask() gives you that control separately.