Suppose you take a conducting sphere and put it in an external field, then the sphere becomes polarised so it has positive charges at one side and negative charges at the other side. However all the charges on the surface must be at the same potential because the potential is the same everywhere in a conductor.
So even though we have both positive and negative charges they are all at the same potential.
This seems weird at first, but it's because we have to add the potential from the external field as well.
For example it means that a field line cannot start and end on the same conductor.