Let's say you signed in somewhere public and forgot to sign out, other people can access your account and cause damage.
Currently, there's no way to sign out of all sessions from the UI, but you can do so manually using a SQLite client.
From your application's root directory, run
$ sqlite3 content/data/ghost.db
This will open your database using the
sqlite3 CLI. From here, run
sqlite> delete from accesstokens;
to flush the accesstokens table and log out of all sessions.
Of course things would be easier if all this can be done on the UI. If you agree, please support this on the Ghost wishlist.