Activity coefficient greater than 1
In chemical thermodynamics, activity (symbol a) is a measure of the “effective concentration” of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.
(From Wikipedia.)

So the main point is that that activity is just a measure of the deviation from the ideal solutions, because the laws you are using in physical chemistry  assume that solution is ideal. But usually it is not true. Laws can work for real life with a certain precision, but you should always remember about their &quot;ideality&quot; and the non-ideality of our world.