What is a no-no?

Jackson’s new favorite word/concept is no-no. He loves to point to things he isn’t supposed to do and say “no-no”. Unfortunately, he doesn’t seem to understand what no-no is supposed to mean. I let Jackson onto the back patio for a moment and when I looked out to see what he was doing, he had picked the last of my tomatoes. He looked at me and instantly said “no-no!”. So he knows picking tomatoes is a no-no, but he doesn’t quite get what that means.