The Window menu deals with the layout of the different windows, and their visibility.

[] Audio/MIDI SetupShows the Audio/MIDI Setup window, where the sound system configuration can be modified
Editor
ShowSwitches to the Editor view
HideHides the Editor, hence showing the Mixer when the windows are attached
AttachIf the Editor window is detached, separated from the main window, attach it back
DetachIf the Editor is attached to the main window, fetach it (makes the Editor a separated window, usefull for multi-monitor setup)
Mixer
Show/Hide/Attach/DetachSame as for the Editor, for the Mixer window
Preferences
Show/Hide/Attach/DetachSame as for the Editor, for the Preferences window
MeterbridgeShows the Meterbridge window, that displays all the tracks' meter at once and their recording status, and is very handy for multitrack recording
ScriptingOpens the Lua Scripting window, allowing to edit and run Lua scripts
[] Tracks and BussesOpens the Tracks and Busses window, which is a shortcut to many tracks/busses operations (routing, effects, …)
[] LocationsOpens the Ranges and Marks window, a single point of control for all range and location markers
[] Binding EditorOpens the Key Bindings window, which allows for easy creation or modification of any keyboard shortcut
[] Bundle ManagerOpens the Bundle Manager window, allowing to create and manage Bundles, which are a way to simplify connection management, by defining groups of ports
[] Big ClockOpens the Main Clock as its own separate (and huge) window, which is helpfull when recording
[] Video MonitorIf a video has been imported in the session, opens a video window (namely, Xjadeo), synced to the timeline
Midi TracerOpens the MIDI Tracer window, allowing to follow each and every MIDI message entering or leaving Ardour
[] Audio ConnectionsOpens the Audio Connection Manager window, a way to make connections to, from and within Ardour's mixer
[] MIDI ConnectionsSame as above, for the MIDI connections
[] LogShows the Log window, where Ardour lists usefull information, warnings and errors