Fisher's Z Transformation?

I have 2 groups of people grasping an object and I'm measuring the force at each finger. Then I'm calculating the force at each finger as a % of the thumb force to get a different percentage at each finger for a total of 100%. In an article I read before they compared the 2 groups they wrote "Fisher's z-transformation was performed on the percentages before statistical analysis", what does that mean? I googled Fisher's z-transformation but can't find anything helpful, at least relating to percentages. What is it and how do I calculate it? Any help would appreciated!