Feathering is the extending and shortening of the edges of the fill in a random manner. This is used in fills that are adding texture to a design, and generally would be used over some other design objects for coverage.
You can adjust independently the left and right inside feathering and outside feathering values. By feathering the edges, you can create simple fur effects.
When setting feather options, realize that left and right are subjective, and if you edit the outline after making this setting, those could actually switch.