Since Japanese gestures and American gestures are so different, it's often confusing when it pops up in anime. The above is a widely popular gesture.
Akkanbe, as seen above, is the act of pulling down one's eyelid and sticking out one's tongue. I'm not sure why this is so, but it's fairly similar to the American gesture of sticking out one's tongue to show disdain or scorn. As in America, it's seen as a very childish gesture.