The Session menu groups together everything related to the session and the file operations.

New…Creates a new session
Open…Opens an existing session
Recent…Opens a list of recent sessions that can be opened
CloseCloses the current session (but not Ardour)
SaveSaves the current session
Save As…Saves to a new session (with options)
Rename…Changes the name of the session
Snapshot (& keep working on current version)…Create a Snapshot but any subsequent change will be saved to this session
Snapshot (& switch to new version)…Same thing, and any subsequent change will be saved to this new snapshot session
Save Template…Saves the session as a template, without the audio
Archive…Exports the session as a compressed file for archiving or sharing purposes, optionally compressing the audio to FLAC
Metadata
Edit Metadata…Opens the Metadata window, where information about the session can be saved
Import Metadata…Creates the metadata by extracting them from another session
Add Track, Bus or VCA…Adds a new track/bus/VCA to the session, same as the Track > Add Track, Bus or VCA…
ImportOpens the Import windows, to add media to the session
Import PT sessionImport a ProTools© session file. Not everything in the original session can be imported.
Scripting
Add Lua Script…Loads or adds a Lua Session script to the current session
Remove Lua ScriptRemoves a loaded Lua Session script from the session
Open Video…Imports a video file in the session
Remove VideoRemoves the video part of the session (the video timeline disappears)
Export
Export to Audio File(s)…Export all or part of the session in audio form
Stem export…Exports each track as its own audio file (for e.g. DAW interchange)
Export to Video FileExports the session to a video file
Clean-Up
Bring all media into session folderCopies all the media files imported from outside the session folder in that folder, see Cleaning up Sessions
Reset Peak FilesReinitializes the buffered images representing the audio files
Clean-up Unused Sources…Quarantines all the media files not used in the session to a specific sub-folder of the session
Flush WastebasketDeletes those quarantined files
[] PropertiesShows the Session Properties dialog, allowing to fine-tune the parameters of the current session
LockLocks the session by showing an Unlock window that (until clicked) blocks every action on Ardour's window
QuitExits Ardour. Prompts for saving the session if it has been modified.