Main authors are persons whose contributions significantly shaped the state of the software at some point in time.
Additional contributors are persons who are not main authors, but contributed non-trivially to this project, e.g. by providing smaller fixes and enhancements to the code and/or documentation.
Of course, this is just a rough overview and categorization. For a more complete overview of all contributors and contributions, please inspect the git history of this repository.
... maybe you?