This tool cleans and refines the edges of opaque objects by either removing fringe pixels or changing their colors to match surrounding pixels.
To use this tool, just hold down the left mouse button and drag the pointer such that the tip area runs over the boundary of the object as shown below:
This tool can be used to clean and finalize cut-outs made using the Background Erase Tool .
You can set the size of the tool’s tip by setting the width
entry on the toolbox. The size of the fringe area is calculated as the product of the width and the level as a percentage rounded off to the nearest integer, e.g. if width is 32 and level is 16, the fridge area will be 32 x 16/100 = 5.12 = 5 pixels.
Options
Clean
This mode changes the color of fringe pixels to match nearby pixels, thus removing halos without changing the size of the object. This can be useful in cleaning soft edges like hair.
Shrink
This mode removes fringe pixels, which has the effect of shrinking the selection slightly.
This tool cleans and refines the edges of opaque objects by either removing fringe pixels or changing their colors to match surrounding pixels. 
Clean
Shrink