What is a Webhook?
A webhook is an HTTP request that one system sends automatically to another when an event happens, such as a payment received, so the receiver reacts at once without polling.
In more detail
Polling asks 'anything new?' every few minutes. A webhook reverses it: the source sends a message the moment something changes. The receiving system exposes a URL and processes the payload, usually JSON. Production use needs a few safeguards. Verify the signature so only the real sender is trusted. Respond quickly and do the work in a queue. Expect retries and duplicates, so handlers must be idempotent, meaning that handling the same event twice changes nothing. Log every payload so you can replay missed ones. Webhooks are the usual trigger for automations that must start within seconds.
Example
Stripe posts a payment_succeeded event to your endpoint. Your automation marks the invoice paid in the ERP and sends the receipt. If Stripe retries, the handler sees the invoice is already paid and does nothing.