When Americans form a circle with their thumb and first finger, with the remaining three fingers pointing upward, they are communicating the message everything is okay or good. This is one of the most recognized hand gestures in the American culture.
However, this same gesture takes on a different meaning in other places throughout the world. In Japan, it may symbolize a coin or money. The French interpretation is zero, nothing, or worthless. This is also the symbol for a private body part in many countries.
The lesson here is to avoid using this hand gesture when traveling abroad unless you know exactly what the local interpretation is.