This is a comma-separated list of the elements to index. Each of these must match an ElasticsearchType subclass under search/elasticsearch/type.
You shouldn't need to change this unless you have implemented a new search type, or want to disable a particular search type.
This setting cannot be changed through the web interface. You may change it by setting a value in your config.php file for the following config variable:
$cfg->plugin_search_elasticsearch_types
.