What is “meta_input” parameter in wp_insert_post() used for?

This part of wp_insert_posts() gives it away:

  if ( ! empty( $postarr['meta_input'] ) ) {
        foreach ( $postarr['meta_input'] as $field => $value ) {
            update_post_meta( $post_ID, $field, $value );
        }
  } 

where we see how the post meta fields are updated/added with update_post_meta().

Here’s the inline description for meta_input:

Array of post meta values keyed by their post meta key. Default empty.

This was added in WordPress 4.4 and here’s relevant ticket #20451 for more information.

Note that using the underscore in front of the the meta key _test will hide it from the custom fields metabox in the post edit screen.