Friction, is a force acting opposite the force acting on a body.
Now , what does this necessary evil mean?
It means that it is a force which is necessary for us, but still an evil or bad.
It is a necessary as it helps us to walk, hold things and to maintain a grip between two objects and rubbing of hands produces heat making warm in winters..
It is evil or bad as it causes wear and tear , wastes energy, produces heat which causes destroying of parts of a machine .
So, a necessary evil..