What does it mean for data to be a outliner

It means that the peice of data is not in accordance with the rest of the data, or it is not vastly different than the rest of the data

I think you mean to ask about a data outlier.

It just means an observation that is far from the other observations. It might be an actual observation, but it also has a high likelihood of being a data error, say a number typed in wrong.

A statistic like the mean is sensitive to the outliers. When Bill Gates walks into a room with fifty people, the average personal wealth goes up by a billion dollars. Robust statistics like the median personal wealth hardly change at all.