Any interaction with a database would be abstracted through a REST API (or something similar).
Models work by default with a REST API, so if you want to retrieve a bunch of records from the database, you would use findAll, which would make an Ajax request to your server. At that point your server queries the database and returns the results in JSON back to the client. Similarily, calling save() on a model instance will make an ajax request to your server and your server-side code would have to interact with the database to create or update the record.
The Cookbook example is a good place to get started: http://javascriptmvc.com/docs.html#!creating
It uses fixtures to simulate Ajax responses so you can run the app without a server-side component. The Ajax service guidelines is a good place to start in terms of figuring out how your server should communicate with you client-side code: http://javascriptmvc.com/docs.html#!services