The outlier is just the number in a set that stands out. For example, if the numbers are 1, 2, 3, 2, 2, 1, 45 the outlier would be 45. However, the outlier could also be closer to the rest.
an outlier is something that doesn't fit in with the group. So if you and 10 other people hang out, and no one ever comes near you, you are the outlier.