The specifics will only make sense if you know a little C/C++, but windowHandle is defined as type IntPtr... which is a memory address. Memory addresses could be absolutely anything, but in this context it will probably be the memory address of a window.
Think of it as a GPS location to the front door of someone's house. If you give a robot this location and tell it to open a door, it will probably open the door. Pick a random location and do the same, it probably won't work out as the robot might end up in the middle of a forest or inside some other building by mistake and there's no door to open. It will try anyways, but the operation will "crash".
Functions such as
BFS.Window.GetWindowByText("Discord") might give you an IntPtr to the Discord window, if the window has that title. Passing that IntPtr to another function that wants a windowHandle will probably work. Like
BFS.Window.Close(someIntPtr)
might close that window. But Discord might only minimize to tray when you try to close its window, so that might not do what you want anyways. (I've never used the standalone Discord app, so maybe it does close?)
