I take it to mean that light behind the screen bleeds through. To check for it take a black photo and go into a dark room and then switch your brightness to high. Look along the edges of the screen for white bleeding coming through the screen.
You'll find many threads about this issue apart from this one. It was a common problem in early iPads but now seems to have largely disappeared. It's merely an inconvenience but Apple were/are (?) replacing them.