He sent it to the contestants in episode 13, "The Sucky Outdoors", as a warning to the campers that they were all in danger. The haiku reads:
> The island is doomed
> Beware the wrath of Chef
> Eat your vegetables
This was a way for Chef to express his frustration with the campers' constant complaining and disrespect for his cooking, but also to subtly warn them about the danger of the island itself, which he had intentionally rigged with various traps and hazards.