The «Feather Edges» option of the selection tools, but you may feather them again with this command.
command feathers the edges of the selection. This creates a smooth transition between the selection and its surroundings. You normally feather selection borders with theEnter the width of the selection border feathering. The default units are pixels, but you can also choose other units with the drop-down menu.
If the selection is on border of the image (or goes beyond the border), this option acts as if the selected area continued outside the image.