When you fret, you worry so much about something that it eats away at you. Many people fret about taking standardized tests, but really, they're nothing to sweat.
Fret comes from the Old English word freton which means to devour like an animal. When you fret over something, it consumes your thoughts. If you tell your mother to not fret about you while you're at a sleepover camp, you're telling her to not worry about you too much. Sometimes it means to be agitated though. When you're waiting for the results of an exam, you might fret and wring your hands. In a totally unrelated meaning, a guitar player calls the raised lines on the neck of the guitar that help him play correctly frets.