This is a measurement of how much of the chocolate is actually made from cocoa bean. It can be both chocolate liquor (ground cocoa bean) as well as cocoa butter. (Cocoa butter is made by pressing the fat out of the ground cocoa bean.) Some manufacturers add cocoa powder as part of this. (We do not, because it results in a chocolate with inferior flavor.) Typically, the remaining percentage is sugar, vanilla (to help round out the flavor), and sometimes a bit of emulsifier, such as soy lecithin (no more than .5%). The higher this number, the richer and less sweet the chocolate typically will be.
How we spoke to the Ecuadorian Congress about preserving the cocoa variety Nacional and against CCN-51