What unprecdented mean?

Something for which there is no similar event or example of before. In recent terms, it is unprecedented that an African-American be elected President of the United States of America.

A precedent relates to something similar having happened before. In court cases, when someone is trying to make a point that is not being accepted, they will often refer to a similar case or cases, where such a thing has happened before. There is therefore a precedent for what they are trying to get accepted, and that helps their case in doing so.