Ok here's the thing...
The Source is the heart of your server, it is composed of many files with C++ codes that makes your server run when compiled. When you compile the source (a.k.a. put all that files together and make .exe files) you get, as output, the accountserver.exe/databaseserver.exe/loginserver.exe/cacheserver.exe and so on. You will edit the source if you know C++ and you want to fix/add stuff in game, like for example, Transmutation System, Colosseum, Consignment Shop and etc.. but you will only succeed if you know C++.
The Resource is where you're going to edit most of things in game, like items, npcs, monsters, drop/exp rates, upgrading chances. To edit those, you don't have to touch your source.
You don't need the Source folder to make your server work, if you have accountserver.exe/databaseserver.exe/loginserver.exe/cacheserver.exe/etc already.
You can even delete the Source folder if you don't know how to code in C++.
If you are really going to make changes in the Source folder, I recomend you to download another server.