It's often used to describe:
* Physical objects: A feather is incredibly lightweight, so anything compared to it is very light.
* "The new laptop was as light as a feather, making it easy to carry around."
* Abstract concepts: The simile can also be used figuratively to describe things that are effortless or unimportant.
* "He took the criticism as light as a feather, not letting it affect his mood."
The comparison to a feather emphasizes the lack of weight and effort involved in handling or dealing with the thing described.