This datasource makes it easy to use Couchbase as a datastore in your application. It takes care of the boilerplate code you'd have to write and makes sure that everything is in place. It also provides convenience wrapper and methods that let you work with your data in a way that is natural to Lithium applications.
If you don't know Couchbase, you'd better head over to the official website first.
Note that it is still under heavy development, but scheduled to be usable when Couchbase 2.0 final will be released. It is designed to work with the latest available ext/couchbase release to support views.
Aside from the basic Lithium requirements, you need to have the PHP extension installed (you can grab it here).
Of course, you also need at least a one-machine cluster of Couchbase running (just install it locally if you're just starting out).