This is the name of the Elasticsearch index that will be used by the search functionality of the site. It is unlikely that you will need to change this from its default value unless you are using the same Elasticsearch cluster as an index for multiple Mahara sites, or if you have done a reindex using the "Bypass index" setting.
If you change this to a new value, you may need to use the "reset ALL indexes" button to trigger the creation of the new index.
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_indexname
.