IndexedDB is probably what's meant, Local Storage (if taken literally) is a simpler and perhaps too limited a solution. There are some options that make IndexedDB easier to use, I've been using PouchDB, which works well. If syncing to Dropbox, you'd then dump the database into a JSON string/file and use the Dropbox SDKs to sync that. If syncing to a database, you can do something more efficient.