A webhook is an automatic notification one system sends another the moment something happens in it - a new order, a submitted form, a received payment. Instead of the receiving system having to ask "did anything happen yet?" (that's called polling), the source system actively sends it a message the instant there's something to report.
In practice, it looks like this: an online store has a webhook set up pointing at your system's address. The moment a customer completes an order, the store instantly sends the order data to that address - and the system can create a job from it right away, with no manual retyping and no delay.
Webhooks are one of the two main ways systems talk to each other automatically - the other is the API, which a webhook is typically used as part of (the incoming side of the conversation).
Related: Webhooks and API: how to connect an online store or website to your system, AI reads the email and creates the job, What is an API, What is MCP.