Activity coefficient greater than 1

From Practical skills in Biology by Allan Jones et al.

At high concentrations of certain ionic solutes, $\gamma$ (activity coefficient) may increase to become greater than unity.

My questions : If activity coefficient $\gamma$ $=$ ${a}/{[C]}$ where $a$ is activity which signifies effective concentration and $[C]$ is concentration, how can $a$ be greater than $[C]$ ?

What is meant by effective concentration is whatever number makes the equilibrium equation work. To a first approximation it is true that, for example, [Na$^+$][Cl$^-$] is a constant in any solution saturated with NaCl at a given temperature, regardless of whatever else might be in the solution; but if you add lots of HCl, although [Na$^+$] does go down as [Cl$^-$] goes up, the product is not exactly constant. You can tabulate values for each concentration that will make the product constant, and these are called activities. As @Gordon Driver mentioned, these values can be either greater or less than the actual concentration, depending on the specific system.