This technique is almost required when using selection masks for photo editing, without feathering hard edges will be clearly visable.
1. open sample image
2. create a selection mask
3. open feather command
4. set a feather radius of 30 pixels
5. invert the selection
6. apply changes, for example reduce the brightness to darken the selection. notice that the edges blend smoothly
7. experiment with different feather values
||30 pixel feathering
||60 pixel feathering|
feathering is an important part of using selection masks. You should experiment with differant feather values as each situation may require slightly different settings.