A webhook is an automated message that one system sends to another the moment a specific event occurs. When a customer completes a payment, when a new subscriber joins your mailing list, when a shipping carrier scans a parcel, the external service does not wait for your platform to ask what happened. It pushes a notification to your server immediately, in real time, without your system ever making a request. Understanding what is a webhook and how webhooks operate inside your platform's infrastructure is essential for any founder building a product that depends on data flowing between systems without delay.