A hex is a magic spell or charm that's meant to cause harm. A witch in a story, for example, might put a hex on a prince that turns him into a chicken.
If you believe in magic, you might either fear hexes or aspire to learn how to put them on your enemies — to hex them. In the Pennsylvania Dutch tradition, "hex signs," forms of stars, flowers, and circles, are commonly painted on barns. Though tourists often interpret these as having superstitious meanings, they're really just folk art symbols. The Middle High German root of hex is Hexe, or "witch."