Plain English Meaning
It's an IFR clearance that lets you pick your own altitude and fly by looking outside, as long as the weather is good enough to do that. You stay on your IFR flight plan, but you fly the airplane the way a VFR pilot would.
Full Definition
An IFR clearance authorizing a pilot operating on an instrument flight plan to fly in visual flight rules (VFR) conditions at any appropriate VFR cruising altitude of the pilot's choice, rather than at a specific assigned IFR altitude. The pilot remains on an IFR flight plan and continues to comply with applicable IFR rules, but selects altitude and maintains terrain and visually using VFR - and .