Complex sentence: The quantum eraser experiment is a variation of Young's double slit experiment establishing that when a photon is acted upon, in any way that permits determining which of the slits it has transited, then it cannot interfere with itself.
Split into simpler sentences: The quantum eraser experiment is a variation of Young's double slit experiment. It establishes that when a photon is acted upon in a fashion that allows which slit it has passed through to be calculated, the photon cannot interfere with itself.