The slightly longer version:
Under normal circumstances, this image, or one similar, is shown at the top of pages on this site:
But instead, you may encounter this image at the top of any page on this site:
If that happens, that means that your browser was unable to load the skyline image. The skyline image can only be loaded over IPv6. So if your computer doesn't have IPv6 connectivity, that can't happen, and the warning image is shown instead.
The point of this setup is to warn visitors that they don't have working IPv6 if they see the warning image, so they know they should take action to avoid problems in the future as IPv4 may no longer be enough to connect to all parts of the internet.
For now having just IPv4 still works, but there are no longer any new IPv4 addresses available, so service providers find it harder and harder to connect new users over IPv4. At some point that will just not work anymore, and at that point we all need to have working IPv6.