Diffraction is an optical effect cause by the light when passing through a small apertures such as f/16, f/22, f/32 etc. and hitting the camera sensor. Every combination between camera and lens has a curtain sweet spot. This means that this particular combination will give the best possible image quality in term of sharpness, details, color and contrast. In most cases it is usually around f/8.
As you can see on the illustration below the light hits a single pixel almost perfectly when set on f/8, on the other hand when aperture is set to f/16 the lights diffracts and spread around on more than one pixel creating mushy looking images with less sharpness and detail.
It doesn’t matter how many megapixels how expensive lens use are using, you will always notice image quality degradation when using small apertures. That is just the law of physics and applies for every camera and lens. Of course, some sensors and lens can handle the diffraction a bit better than others, but even with the most expensive ones you will start seeing the effect beyond a certain point.