Soften Normals is a convenience SOP that is frequently used to prepare geometry for baking. The tool softens all your normals, and also allows you to harden the normals for the vertices part of the uv seams.
Specify a uniform cusping angle to be applied to the geometry its normals. By default this value is set to 180, which will produce a very smooth look.
Harden UV Seams
This toggle instantly hardens the normals of the vertices part of the uv seams. This is a very important step for baking textures to prevent artifacts around the uv seams.
This specifies the cusping angle for the normals of the vertices part of the uv seams. This option is only enabled when “Harden UV Seams” is enabled.