A 'bump' is an area that is raised/higher than the surrounding area. For example, a 'speed bump' is a small part of the road that is higher so that car drivers reduce speed before driving over it (to slow down the speed of cars). If you are bitten by a mosquito you might have a bump on your skin. The adjective 'bony' is used to describe things that are made of bone, or like bone. In this case a small part of the skull is not flat and appears to have more bone. Therefore it is described as a 'bony bump'.