Fretting is the wear that occurs in two surfaces which are supposed to be stationary.  In certain conditions, very slight motion may occur between surfaces which are otherwise stationary.  This can be due to vibration or due to dimensional changes due to temperature variations.

If the change is repetitive, there can be wear along the surfaces.  This can, sometimes, be accompanied by chemical reactions, such as oxidation, which may occur along the surface.  This is known as fretting corrosion.

The worn out materials may get trapped between the surfaces and can cause further damage.

Fretting can be prevented by reducing micro-motion between the surfaces.  Special additives added to lubricants can also minimize fretting.