Yes. On a plane or a train, keep working as usual:
Your notes are stored on the device, so you can open, search, and edit them without a network.
Your edits are saved on the device while you're offline.
When you're back online, Granite sends them and fetches what changed elsewhere.
The web app also works offline, as long as you've opened it on that browser before.
If you and someone else changed the same lines while apart, Granite keeps both versions. See What is a conflict copy?