Register for COM Interop

Allows your managed .NET assembly to be accessible from an unmanaged COM library
This option will cause Visual Studio to automatically register your assembly as a COM component in the windows registry when the project is compiled.
Registering for COM Interop requires administrator permissions.
You must be running Visual Studio as an administrator

If you don't check this option you can still generate a COM wrapper by using tlbexe.exe and register your library manually by using regasm.exe
You must also configure the Setup project to register the assembly for COM Interop when it is installed.
In the properties window for the primary output make sure that Register is set to vsdrpCOM.

This property is not available for Windows Applications or Console Applications
This actually runs the RegAsm after every build to register the types with COM
Every time you change the assembly methods a different GUID number will be generated
This is only necessary if you are building a standard class library COM object
RegAsm - registers the file and creates a corresponding type library

Microsoft Office applications are COM Servers and communicating between Office and .NET code is known as COM Interop.
All the COM components and Win32 API functions are unmanaged code.
All .NET code is managed code.
There are a large number of different ways to combine the features and power of .NET with Microsoft Office

These rely on special interfaces (or protocols) and are very focused in design.
They include the following: COM Add-ins (Managed), Real Time Data Components, Smart Tags, Smart Documents and Visual Studio Tools for Office