[lottin]

M2 is a measure of the money supply. There are different measures of the money supply, which roughly speaking are M0 (cash), M1 (M0+current accounts), M2 (M1+savings accounts) and M3 (M2+money market instruments). The fact that they have divided M2 by the population seems a little strange, but basically the graphic shows the amount of "money" (cash+current accounts) per person over a time period in the US.

[SigmundA]

Per Capita is a pretty common way to normalize a statistic to account for population growth / change right?

[lottin]

Yes, but the money supply is rarely reported in per capita terms. (It's not that it doesn't make sense, it's just unusual.)