What are the main folders inside the MonoDevelop Projects

Hey, I am very new to Unity. I don’t understand what the MonoDevelop Projects Drawer Folders do that have those “special” icons. The most confusing part is, why do I see some of the same files/folders inside them?
Thank you.

Firstpass is your compile first assembly, this includes that scripts(C#/JS) in the Standard Assets(Please see special folders from the manual) that need to be build first/in a particular order. The asset location is the same as the editor extensions(Assembly-CSharp-Editor) and normal assembly(Assembly-CSharp) so they can contain the same files/folders but they may not have the same entries as you can clearly see. You can see your Scripts folder is excluded for the Editor and firstpass assembly, this is because there are particular folders(see the special folders link above) that need to be included for those assemblies but not your general scripting assembly. The references folder(s) are assembly references which can determine the assembly build order. Meaning if a reference refers to another assembly then that other assembly must exist/be build first. This chain can go on as long as the references are referencing other projects and there assemblies/namespaces/classes/etc.