What is a webhook?
In one sentence: A web address one server gives another so it can be called the moment something happens, instead of checking over and over.
A little more
A webhook is a web address that one server gives to another, saying: call this when something happens. When a payment goes through, a form is filled in or new code is uploaded, the first server sends a request to that address straight away, carrying a small description of what happened.
Webhooks save the second server from polling over and over. Because anyone who learns the address could call it, real webhooks are usually signed with a secret so the receiver can check the message really came from the right place, and senders retry for a while if the receiver is down.
It's a bit like… Leaving your phone number with a shop so they ring you when the thing you ordered arrives, instead of you phoning them every day.