Diffraction is an optical effect that limits the total resolution of your photography, no matter how many megapixels your camera has. Diffraction occurs because light begins to disperse or diffract when going through a small opening such as your camera's aperture.
Here are some videos about this topic:
In this video by Tony Northrup gives a great walkthrough on this topic and the science behind it.
In this video, Steve Perry outlines everything you need to know about lens diffraction. From what diffraction is to how it applies to photography
The video is loaded with practical examples that answer questions like:
- How far can you safely stop down?
- Does diffraction limit the usefulness of high res sensors?
- Can you sharpen out diffraction?
- Should you ever use small F/stops?
- What causes diffraction in the first place?
- What's the difference between sharpness and depth of field?