The filter draws black parts in the image restoring them according to the surrounding image.
This transformation eliminates shortcomings of the image (black parts, appeared by capturing, damaged areas of photos and so on) with the help of the adjacent areas of this image. As the result of the work of filter the restored area hardly visible from the rest.
The Radius (inpaintRadius) determines the radius of the area around the parts according to which the image is built. The more is this parameter — the slower works the filter and the more precise the empty area is drawn.
Please find more information on settings and the use of filter in the step by step instruction Tutorial on the use of Inpaint image without mask filter.