What is voltage difference?

Voltage at a point is defined as the work done by a unit positive charge carrying it from infinity to that point. while volage diffrance between two points is the work done bye unit positive charge from one point to the other point.

work may be positive or negative depend upon the nature of charge that's why the potential will be negative or positive .

By definition, voltage means potential difference. For this reason, you cannot have 'a voltage at a point' because potential difference, or voltage, exists between two points. You can, however, have a potential at a point.