Complex sentence: With the addition of quantum theory, though, the electromagnetic potential is seen as being more fundamental or "real"; the E and B fields can be derived from the potential, but the potential cannot be derived from the E and B fields.
Split into simpler sentences: With the addition if quantum theory, though, the electromagnetic potential is seen as being more fundamental. The E and B fields can be derived from the potential, but the potential cannot be derived from the E and B fields.