What does indices.indexing.index_current counter mean?

Elastic Stack Elasticsearch
1

I user 'GET /_nodes/es-diag-20/stats' to get the states of a node, does anyone know what 'indices.indexing.index_current' counter mean? For example :

        "indexing": {
           "index_total": 452856851,
           "index_time_in_millis": 1566958538,
           <h3>"index_current": 2726089, </h3>
           "delete_total": 0,
           "delete_time_in_millis": 0,
           "delete_current": 0
        }

Seems that it means the indexing operations are occurring on this node, but that's too huge to me. Any idea?