Plain English Meaning
The weather briefer is telling you that the weather is bad enough that flying this trip under visual rules looks unsafe. They are not refusing to brief you and they are not grounding you — they are warning you, and the choice is still yours.
Full Definition
An advisory statement issued by a Flight Service briefer when, in their judgment, current or forecast weather along the proposed route makes Visual Flight Rules (VFR) operation hazardous. The phrase is given when sky conditions, visibility, or other weather factors are at or below VFR minimums, or are expected to deteriorate to that level during the flight. It is an advisory only; the decision to fly remains with the pilot in command.