Minification means removing all the unnecessary characters from the source code without changing its functionality. These characters are needed to add readability to the code, but are not required for it to execute.
Minified source code is not recommended for development because it makes code almost impossible to read and edit. When development problems arise, you want to be able to see the source and try small changes which is much easier to do with unminified source code.
However, minified source code is recommended to be used in production due to its smaller size and download speed.