Something that is unprecedented is not known, experienced, or done before. If you've never gone on a family beach vacation but you're planning one now, you could refer to it as an unprecedented decision.
The root of this word is precedent, a noun referring to something done or said that is used as an example to be followed in the future. In law, a precedent is a legal decision that is used as a standard in future cases. So the adjective unprecedented, meaning "having no precedent," was formed from the prefix un- "not," the noun precedent, and the suffix –ed "having."