The median is called a resistant measure, whereas the mean is a non-resistant measure. What's a resistant measure?

Resistant in this case means it can withstand extreme values.

Example: Imagine a group of 101 people who have an average (= mean ) of $1000 in the bank. It also happens that the middle man (after sorting on bank balance) also has $1000 in the bank. This median means, that 50(%) have less and 50 have more.

Now one of them wins a lottery prize of $100000, and he decides to put it in the bank. The mean will immediately go up from $1000 to near $2000, as it is computed by dividing total amount by 101. The median ("middle of the row") will be undisturbed, as there will be still 50 with less, and 50 with more money in the bank.