About the Feather Concept
Feather is a float value in points (or whatever unit you have defined in File > Setup > Misc > Unit System) that represents the width of the area that is going to be dissolved at the edge. Feather is the light horny waterproof structure forming the external covering of birds. See English Feather definition.
The Feather parameter is an standard parameter for the following types of layers:
Strangely Rectangle Layer doesn't have the Feather parameter.
When you apply a Feather to any of those layers, then the edge of the shape becomes dissolved and spread out and shrunk the amount that the Feather value indicates.
|Circle with Feather set to 0 points||Circle with Feather set to 20 points|
Feather's Complementary Parameters
There is another parameter that works together with the Feather parameter, and which specifies the type of feathering to be used. On Circle layers the parameter is called Fall Off and for the other featherable layers it is called Type of Feather.
Type of Feather
|Fast Gaussian Blur||Gaussian Blur||Disc Blur|
|Box Blur||Cross-Hatch Blur|
That's the type of feather for the Circle Layer. All the example images below have the same amount of Feather (set to 20 points).