What does 60 % cocoa or any other percentage of cocoa mean ?

The cacao percentage is the percentage of the ingredients that come from the cacao tree, including both the fat, i.e. cocoa butter, and the cocoa mass and eventually added cocoa powder.

A chocolate with 60% cocoa refers to the sum of all the cocoa ingredients in the recipe (the amount of cocoa mass, cocoa butter and cocoa powder in the recipe).