The runtime-callable-wrapper (RCW) is a .NET technique for COM interoperability. RCW wraps a COM object so that the .NET runtime system can call it. It effectively exposes a COM interface so that managed code can call it. For more background, see the Visual Studio Help topic Runtime Callable Wrapper.
To use an RCW, first you package the COBOL as a COM object and register it.
You then add the COM object as a reference in your project. When you build the project, the RCW is generated, thereby exposing the COM class as a managed proxy class.
The benefit of using RCW is that your managed program just sees the COM object as a managed class (and you get the benefit of IntelliSense). The .NET run-time system manages the data type manipulation to convert .NET types to COM types for you.