This tool is used to convert debugging symbols generated by Microsoft's compilers into debugging symbols that can be consumed by Mono's runtime.
Program Database files are files that end with the extension .pdb and are associated with a library or executable (a .dll or .exe file). This format was until recently not documented so Mono used its own file format for storing debugging information, the Mono debugging format.
Just like PDB files Mono Debugging information is stored in files that reside side-by-side with a program executable or a library.
You can use the pdb2mdb tool to transform these PDB files into MDB files that Mono can then consume to debug programs or provide line-number information for stack traces or exceptions.
The pdb2mdb tool is released under the terms of the Microsoft Public License (MS-PL). JB Evain wrote the tool. The PDB reading support came from Microsoft Common Compiler Infrastructure.