typedef for a string that describes the "source" of a diagnostic. If you are working on a large project with many small code modules you may only want to enable debugging from particular source modules. This typedef facilitiates this.
Depending on the desired granularity of your dbg sources you will use different naming conventions. For example, your dbg_sources might be filenames, that way you can switch off all debugging output from a particular file quite easily. It might be device driver names, component names, library names, or even function names. It's up to you.
If you provide the DBG_SOURCE macro definition at compile time, then the DBG_HERE macro includes this source name, differentiating the sources for you automatically.