Someone who's frightened is scared or anxious. A frightened camper might tremble with fear as she listens to the scary campfire stories her counselor tells.
When you're frightened, you feel fearful — you might be frightened of thunderstorms or frightened of strange dogs. A frightened airplane passenger might panic and cause other people to become frightened. The adjective comes from frighten — before the 1660s, the verb was instead fright. All of these words share an Old English root, fyrhtu, "fear, dread, trembling, or horrible sight."