A thing Dutch people comment whenever they see anything remotely Dutch anywhere on
the internet.
Gekoloniseerd literally means 'Colonized', referring to the colonial history of
The Netherlands. The suggestion is thus that similarly to how the Dutch spread across the globe to colonize far-away lands that they are still spreading, but this time across
the internet. One can also view it as a simple statement of "The Dutch were here."
This tradition most likely started on the subreddit r/cirkeltrek on Reddit. Reddit is still the place one is most likely to encounter a comment containing GEKOLONISEERD.
Gekoloniseerd is often spelled in such a way to draw as much attention as possible, with the aim to attract other Dutch visitors. e.g. "G E K O L O N I S E E R D". It therefore also isn't common for such a comment to start a chain of Dutch reactions and memes.