What are "aggregators in metricsSpec" supposed to mean?

In my opinion, “metrics” should be a single integer or float in the data schema, and how does this related to “aggregators”?
When does the “aggregate” happen when we building index.

Hi Kurt, Druid rolls up data at ingestion. This doc may help: http://druid.io/docs/latest/design/index.html

Thanks, it’s really help