What is the difference between activity and concentration in electrochemistry?

Activity is the "idealized" concentration, assuming that the molecules of a solute are not in each other's way.

If you double the concentration, ideally reaction speeds would double. In practice, especially at higher concentrations this is not exactly the case. The (re)activity of the doubly concentrated solution is slightly less than double. This is what is represented by the activity coefficient, and the corrected concentration is called the "activity" of the solution.