Something that's exposed is uncovered or unprotected. An exposed hillside isn't sheltered by hills, trees, or buildings, but is completely open to the wind and weather.
If you venture out on a bitterly cold day with no gloves, your exposed fingers might get frostbite, and a soldier in battle with no body armor is dangerously exposed. If you leave a wooden chair outside all year long, exposed to rain and snow and sun, it will eventually fall apart. Exposed comes from the verb expose, "leave without shelter or defense," from the Latin root exponere, "lay open or reveal."