Essentially there is too much moisture in the cookies. This can happen in a couple of different ways. First, you may have accidentally added too much food coloring/extract liquid. This is why I much prefer the gel food coloring since it doesn't add extra liquid. Secondly, this can also happen if you make the meringues in a humid environment. And lastly, the cookies needed longer in the oven (either during the baking process or leaving them in the oven to partially cool). To counteract that, simply add them back to the oven for another 30-60 minutes, until they're dry, but not browned.