LB: "Average", when most people think of it, is the mean value (total value divided by number of entries). The median value is, if you sort the list from lowest to highest, the value 50% of the way along the list.
An illustration: you have five people, labeled A through E, with the following wealth:
A: $1 B: $2 C: $3 D: $4 E: $9990
The mean wealth of this group is $2000 ((1+2+3+4+9990)/5).
The median wealth of this group - the value halfway along the list - is $3.