Making halftone dots smaller in color correction and platemaking by adjusting film exposure time.
This is sometimes done to offset for the dot gain that occurs when being printed on the press. Sharpening is also done electronically by exaggerating the difference between tones or colors at their edges.
Some paint and color manipulation programs have tools to sharpen selected areas of the image rather than having it affect the entire image.
Sharpening is helpful when the image only needs touching up in specific areas.