Some event(s) happens in Akeneo PIM. This triggers a mechanism to send those event(s) as HTTP POST request to your Request URL. Each request contains event, with correspondent event type presented in JSON format. Events API sends one request per one event, and sending of requests happens in real-time.
If Akeneo family contains variants, then during family update (or it's variants as well), Akeneo will re-save related products. It will trigger sending product_updated events.