What does increase the effect of farm improvements mean?

I believe you are talking about a skill for a hero, possibly The pioneer.

Her second skill is called 'Farm Improvement' and gives a boost to the production rate of farms. The 4th skill is 'Pursuit of Perfection' and says that it increases the effect of Farm Improvement. So it will further improve the effect of the 2nd skill. Adding even more output. Other farm production heroes have the same skills as well.