These are typically very large, round, signs that are colourfully-painted that are found on barns, houses, and other farm buildings by the Pennsylvania Dutch people of the northeast United States. The hex signs were originally intended as a defense against hexerei, or witchcraft spells, but are now used mostly for their distinctive aesthetics.
Each design is enclosed in a circle and has a meaning. The most common designs utilize five, six, and seven pointed stars, hearts, the solar wheel, flowers, and farm tools. They are painted in primary colors. Little more is known as it is a taboo for these folk to talk about them to outsiders.
The custom of using hex signs on barns but not houses comes from Germany. The custom of using hex signs on houses and not barns comes from Switzerland. The Pennsylvania Dutch (in the USA) simply combine both customs and paint hex signs on whatever farm building they wish.