[ edited excerpt from developer's www site ]

During image processing operations such as scanning and scaling,
images tend to get blurry.

The blurred impression of these images is due to the fact that image
pixels are averaged with their neighbors. Blurred images don't have
sharp boundaries and look as though they have been taken with an
unfocussed camera.

Refocus is a GIMP plug-in to "refocus" the image, using a technique
called FIR Wiener filtering. The traditional technique for sharpening
images is to use unsharp masking. Refocus generally produces better
results than unsharp masking.

The plugin can be found under "Filters > Enhance > Refocus"