Popularized by Pennsylvania Dutch farmers who painted their barns and outbuildings with circular stars and other patterns. Hex signs and circles are related to the German traditional art of fraktur, an ink or water color style of lettering and symmetrical and elaborate folk art. Hex circles usually appear in the form of stars inside circles, but designs may include flowers, such as roses, birds, hearts, tulips, and trees. Hex signs on the barns became popular tourist attractions. Some believe the signs have supernatural meanings among the original Pennsylvania Deitsch or Pennsylvanian German denizens.
Learn more about Hex designs here at the Wikipedia page on German Folk Art.