Plain English Meaning
Something has gone wrong with one of the glass cockpit screens, or with the equipment feeding information to it, so the pilot can no longer trust what it shows -- or part of it has gone blank.
Full Definition
A failure or degraded condition in one or more components of an electronic flight display (EFD) system, in which the primary flight display, multi-function display, or the sensors and computers feeding them no longer present accurate, complete, or reliable flight information. Malfunctions can range from the loss of a single piece of data (such as airspeed or attitude) to a full screen failure, and may be flagged by the system with warning indications, red Xs over affected data, or comparator alerts between independent sources.