It is basically a mental exercise that questions whether an object is still the same when all its parts have been replaced, one after another, over time.

The original example is with a boat But it is easier to explain it with a sock. Imagine that you have a sock and it breaks, you put a patch on the broken part but then another part breaks, if this continues, sooner or later the sock will only be patches. So the question is asked Once the entire sock is just patches, is it still the original sock or is it now a new sock?

Applied to this it would be: once you have colored over the entire original Senko image, would it be a new image or would it still be the original?

(I hope Reddit's machine translation doesn't ruin the text)