Complex sentence: The fine-structure constant is the coupling constant of quantum electrodynamics (QED), although it is not really a constant owing to the zero-point energy of the electron-positron field.
Split into simpler sentences: The fine-structure constant is the coupling constant of quantum electrodynamics (QED). It turns out that the fine structure constant is not really a constant at all owing to the zero-point energy of the electron-positron field.