When performing a \'svn merge\' from my development team\'s trunk into a branch, we occasionally experience merge conflicts that produce files with suffix names: *.mer
It seems that the "left" file is the last version where the file was the same in the trunk and the branch (for your question, but it would be between the source and dest in general).
If you've never merged changes from the trunk into your branch then this would be the version of the trunk when a branch (copy) was performed. Otherwise, it's the last version of the trunk that was merged and committed to this branch.
The left and right are what is used to create the diff that will be applied (as a patch) to the working file.