A Call a Webhook step means that whenever that trigger event occurs, the webhook will be invoked to collect the data and send it to the URL you specify.
For example, an hour after someone registers and pays for an event using the Invoice Details and Payment Widget, a process calls a webhook to determine whether the invoice has been paid. If it has not been paid within the hour of creation, the invoice and associated Event Participant record are marked as "Abandoned". If there was a limit on the Product Option Price, the abandoned invoice will not count toward that total.