Whilst reducing the aperture of a lens usually reduces aberrations and hence improves the flatness of the MTF, there is an optimum aperture for any lens and image sensor size beyond which smaller apertures reduce resolution because of diffraction, which spreads light across the image sensor.
This passage explains that while reducing the aperture of a lens generally improves image quality by minimizing aberrations, there is an optimal aperture for each lens-sensor combination beyond which further reduction in aperture leads to a decrease in resolution due to diffraction.