pH, conc vs activity

After studying a somewhat overview of "gamma" activity coefficient (it is not actually in our course but may come in exam as a part of unknown paragraph in ionic equilibrium) I can understand most things , but can somebody please just explain in layman's terms (so i can understand it logically, not experimentally) how ionic strength can increase the coefficient and how is distance of approach (lost here) comes into play.


{I am familiar with laplace equations but by only using fourier tricks etc, so, i cant solve any general question with my knowledge}


2. Are there any more very well known effects only to be understood by activity like 
ex- When a 0.1 M hydrochloric acid solution is added to 5 M solution of magnesium chloride, pH decreases — when in fact the acid has been diluted.


