From the documentation:
If the parameter value is prefixed with @ then the value of that parameter is extracted from the data object (useful for non-GET operations).
And later on they have an example
var User = $resource('/user/:userId', {userId:'@id'});
So the second argument to the resource service are the default parameters. In this case, by default, the userId will be extracted from the resource object you call a method on.
Previously I had an example of a GET operation, which doesn't really make sense in this case. But imagine you were doing a POST ($save) request with a data object. Angular would automatically extract the userId from the data object and inject it into the URL for you. You don't have to manually specify the value for the URL.
More info can be found on the doc page.
http://docs.angularjs.org/api/ngResource.$resource