A source map is a file that maps from the transformed source to the original source, enabling the browser to reconstruct the original source and present the reconstructed original in the debugger.
Source maps are often requested when DevTools is attached and source code contains a comment like this:
However, note that unlike other sources loaded along with the main document, source maps are requested without the referer header. This is not a problem in most cases, unless your web server filters incoming requests on referer.
After fetching the source map, this original source file(s) specified in the source map will be fetched, and finally get presented in DevTools.
Hope this info helps when you fail to make source mapping work.
Update (): If you have difficulties to make source mapping work, try using Firefox, as it logs source map related errors. See Source map errors - Firefox Developer Tools | MDN.