Plain English Meaning
ATC lets you stay on your IFR flight plan but allows you to pick your own altitude, as long as you can stay clear of the clouds by the visual flight rules and see well enough to fly visually.
Full Definition
An ATC clearance authorizing a pilot operating on an IFR flight plan to maintain VFR cloud clearance and visibility requirements while flying above, below, or between cloud layers. The pilot remains on the IFR flight plan and continues to comply with all IFR rules except for altitude assignment, instead selecting an appropriate VFR cruising altitude.