What is the _sources folder used for ?

I think the normal (non-source) folder is the minified and compiled version of the _source folder. _source is absolutely uncompressed and can be used to develop plugins etc for CKEitor. While developing you can simply include the ckeditor_source.js file instead of ckeditor.js and _source files will be used instead of the normal files. However, for production mode to improve loadingperfomrance, it's better to use the minified and compiled version. There's a tool to compile your [modified] _source to a production-ready ckeditor