Symbol files are created when images are compiled and are used for debugging an image. They allow someone with the correct tools to view code as the software is running. In addition, when an exception occurs the symbol files allow the line number of the issue to be determined and added to the stack trace.
Whilst symbol files are generated automatically for debug versions, it’s entirely possible to generate them for release versions as well.
Since 5.2.0 DotNetNuke has made release debug files available, and these can be installed into production sites to help pinpoint the line where an issue occurs in the stack trace.
Note: to create release symbol files for your own projects see here