An Applcation Reference [AppRef] is a reference to a VI Server, which can operate on the local machine or somewhere on the (global) network.
The Open Application Reference Node on the Application Control Palette returns a reference to a VI Server Application that runs on the specified computer. If no computer is specified, it defaults to the local machine.
The returned application reference can be used in conjunction with the Property and Invoke Nodes to get or set properties and invoke methods on the application, including opening a VI Reference.
The AppRef should be closed with the Close Reference function, if it is no longer required, allthough LabVIEW will close it as soon as no instance uses it any longer (usually when the top-level VI of the calling chain to this AppRef goes idle). An AppRef is also used to open a VI-Ref using the Open VI Reference node in the very same palette.