A webhook is a way for one system to push real-time notifications to another. Instead of System B repeatedly polling System A’s API (“anything new?”), System A makes an HTTP POST to a URL System B has registered, with details about the event (“new order created”).
For customer portals, webhooks are essential for keeping the portal in sync with backend systems: when a new invoice is created in the billing system, a webhook notifies the portal so the customer sees it immediately. When a support ticket is updated, a webhook updates the portal. Without webhooks, integrations rely on slower, less responsive polling.