You can smooth the hard edges of a selection
by feathering. Feathering blurs edges by building a transition between
the selection and surrounding pixels. This blurring can cause some
loss of detail at the edge of the selection.
You can create a feathered selection with the Elliptical
Marquee, Rectangular Marquee, Lasso, Polygonal Lasso, or Magnetic
Lasso tool. You can also add feathering to an existing selection
by using the Select menu. Feathering effects are apparent when you
move, cut, copy, or fill the selection.

Original image (left), and after making a feathered selection,
applying the Inverse command, and filling with white (right)