Sources are used to search documents in external APIs such as Google Books. A source is based on two Javascript scripts: Search and Document. For a real example, take a look at the Google Books source included in the Books template.

Search script

The Search script returns a list of results based on the user input. For example, when you search a book by title, the script sends a request to the Google Books API and returns a list of results that will be presented to you.

Document script

The Document script inserts a new document in the collection. It receives one or more parameters from the Search script, sends a new request if needed, and insert a document.

Library script

The Library script is an additional script shared between the Search and Document ones. You can put your classes or functions in it.

Documentation

You can find the full documentation at this page: Javascript API.