Question

What is meant by saying that the potential difference between two points is 1 V?

Answer 1

If 1 J of work is required to move a charge of amount 1 C from one point to another, then it is said that the potential difference between the two points is 1 V.

Answer 2

The potential difference between two points is said to be one volt if one joule of work is done in moving one coulomb of electric charge from one point to the other.
Thus:
1 volt = `(1  "joule")/(1  "coulomb")`