Debye-Huckel limiting law
$I$ is the ionic strength of a solution, and is a measurement of the concentration of ions in a solution. $m_i$ is the concentration of the ion i, and $z_i$ is the charge of that ion. The $\frac{1}{2}$ in front of the summation is a result of the fact that both positive and negative ions are considered when calculating the ionic strength. $m_i$ could either be a measurement of Molarity or molality, and your book should state which definition you are using (I assume molality due to the lowercase m).