A webhook is a way for a system to tell another system about an event straight away. Instead of repeatedly asking whether anything has changed, the second system receives a message when it does, such as a new order, a signed document or a paid invoice.
This makes connected systems faster and less wasteful. A payment provider can notify your system the moment a customer pays, so the job can be released without someone checking. Webhooks need to be secured and monitored, so that forged messages are rejected and failed deliveries are noticed and retried.