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 session that can be opened
Close
Closes the current session (but not Ardour)
Save
Saves 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, optionnaly compressing the audio to FLAC
Metadata
Edit Metadata…
Opens the Metadata window, where informations 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…
Import
Opens the Import windows, to add media to the session
Import PT session
Import 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 Script
Removes a loaded Lua Session script from the session
Open Video…
Imports a video file in the session
Remove Video
Removes 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 File
Exports the session to a video file
Clean-Up
Bring all media into session folder
Copies all the media files imported from outside the session folder in that folder, see Cleaning up Sessions
Reset Peak Files
Reinitializes the buffered images representing the audio files
Clean-up Unused Sources…
Quarantines all the media files not used in the session to a specific subfolder of the session
Flush Wastebasket
Deletes those quarantined files
[] Properties
Shows the Session Properties dialog, allowing to fine-tune the parameters of the current session
Lock
Locks the session by showing an Unlock window that (until cliked) blocks every action on Ardour's window
Quit
Exits Ardour. Prompts for saving the session if it has been modified.