Plain English Meaning
ATC has cleared you to fly at a VFR altitude of your own choosing while still on your IFR flight plan, as long as you can stay clear of clouds and have the visibility VFR requires.
Full Definition
An ATC authorization for an IFR aircraft to operate in visual flight rules (VFR) conditions at any appropriate VFR altitude (as specified in the regulations) above, below, or between cloud layers. The pilot remains on an IFR flight plan and continues to comply with all IFR rules except for altitude assignment, choosing instead a VFR cruising altitude that maintains required cloud clearance and visibility.