This is a new technology available in some of the more recent cameras, and particularly in the mirrors cameras like the Sony a7. Its an alternative to DOF preview, whereby the camera can tell you which part of the image is considered to be in focus by flashing an overlay on those pixels that are in focus.
On a mirrorless camera where the viewfinder is electronic, it can be more helpful than the traditional DOF preview because the viewfinder does not get dimmed down when the function is activated.