to simplify, SourceMap allow us to live edit or debug compiled and compressed files like sass, coffeescript from the main source file.
This is how you do on chrome,
Open up your Developer Tools and click on settings, make sure that the option: Enable css “Enable CSS source maps” and “Auto-reload generated CSS” are both ticked.
Now you need to map your css to sass or scss file,
Open the Source tab from developer toolbar and right click the css you want to map to the scss