The code composes a mailto command as in (all on one line):
mailto:firstname.lastname@example.org?Subject=You must buy these books!
&Body=Prototyping with Visual Basic%0D%0AVisual Basic G
raphics Programming%0D%0AReady-to-Run Visual Basic Algo
The HttpUtility.HtmlEncode method converts a string into an HTML-encoded string, replacing special characters with hex values and so forth.
Next the program makes a ProcessStartInfo object specifying IExplore.exe as the program to start and the mailto command text as an argument for the program. Finally it uses Process.Start to start IExplore.