ReturnCode handling? msi installation

Hi im currently trying to understand how to use returncode for error handling in a script.
Belov i have a script that install iff the registry key doenst exist.
The part i dont understand is this section. Anybody who could explain line by line what happening or point me to a site where i can read about this. 
Thanks
  ReturnCode = oShell.run (CHR(34) & sRunPath & "Prerequisite Components\Microsoft Report Viewer 2010\ReportViewer.exe" & CHR(34) & " /passive /norestart", 0, TRUE)
  If ReturnCode = 3010 Then ReturnCode = 0
    If ReturnCode <> 0 Then
      Set oFSO = Nothing
      Set oShell = Nothing
      WScript.Quit(1)
End IF


' --- Microsoft Report Viewer 2010 x86
RegEntry = "HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\{72DEBE5A-5667-3966-8E8D-2FD5FBCCB7DD}\UninstallString"
If Not regEntryExists(RegEntry) Then
  ReturnCode = oShell.run (CHR(34) & sRunPath & "Prerequisite Components\Microsoft Report Viewer 2010\ReportViewer.exe" & CHR(34) & " /passive /norestart", 0, TRUE)
  If ReturnCode = 3010 Then ReturnCode = 0
    If ReturnCode <> 0 Then
      Set oFSO = Nothing
      Set oShell = Nothing
      WScript.Quit(1)
    End If
End If

Function regEntryExists(RegEntry)
  Dim Entry
On Error Resume Next
'set shell = CreateObject("WScript.Shell")
  Entry = oShell.RegRead(RegEntry)
  If Err.Number <> 0 then
    Err.Clear
    RegEntryExists = FALSE
  Else
    Err.Clear
    RegEntryExists = TRUE
  End If
'set oSshell = Nothing
End Function

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