How to interpret arg min in the the following equation?

I am studying the following equation:

$\hat{s}_m(n) = \text{arg}\text{min}_{s_m(n)\in A_s}|\frac{\psi_m^H}{||\psi_m^H||^2}y_m(n)-s_m(n)|^2$----(1)

here $A_s$ is 1x$N$ vector of QPSK symbols, $s_m(n)$ belongs to $A_s$, $\psi_m$ is a random complex number, $y_m(n)$ is 1x$N$ vector and $n$ ranges from $1$ to $N$ and I have all these values.

My query is what does arg min is signifying in this equation (1).

Any help in this regard will be highly appreciated.