A webhook is an automatic message one service sends to another when something happens. For example, a code hosting site might notify a project tool when someone adds new code, so the project tool can update itself.
It is like asking someone to ring your doorbell when a delivery arrives, instead of checking the window every minute. The receiving service needs a web address that can accept the message. Because the message comes from outside, the receiver should check that it really came from the expected service before acting on it.