One outlier moves the mean

An outlier is a value far from the rest. It might be a genuine extreme, or it might be a typo. Somebody keying 1200 when they meant 120, and the two look identical in a column.

The mean uses every value's size, so one extreme figure pulls it. The median only cares about position, so unless the outlier changes which value is in the middle, the median does not move at all. That resistance has a name: the median is robust.