'sharpen' applies sharpening to an image, to emphasize edges within the image. It reduces blur and gives greater focus to the image details. The range is set on a scale from 0 (default) to 100.
A sharpen value of 1 is usually sufficient to significantly emphasize edges.
Sharpening has the benefit of reducing file size. However, it overrides the quality option ("unsharpening" would otherwise be applied during image compression), so you will see no difference in quality be it from 0% to 100%.
Compare these images, with and without sharpening: