Someone who's rattled feels confused and upset. If you're easily rattled, you might want to think twice before taking a job as a lifeguard at a busy swimming pool.
When you're rattled, you're shaken and maybe a little fearful. Everyone gets rattled sometimes — you may feel rattled when you realize it's your turn to give a report in class and you're completely unprepared. Others feel rattled when their phone battery dies and they're left navigating an unfamiliar city without a map. This figurative, informal adjective has been used in English since the mid-19th century.