Refnums in LabVIEW are not a new thing - they represent some external resource that must be tracked and released when you're finished with it. The most common example is the file refnum, created whenever you open a file.
For those programming .NET in LabVIEW, refnums are your constant companion. We convert what .NET data types we can into native types, but there are simply too many types in .NET that have no correlation. So, you get refnums.
But, just like files, you need to be sure to close the refnum when your done with it, or you'll get a memory leak. Let's take a look at when you do and don't need to close a .NET refnum.