By Wikipedia, a Webhook is:

A webhook in web development is a method of augmenting or altering the behavior of a web page, or web application, with custom callbacks. These callbacks may be maintained, modified, and managed by third-party users and developers who may not necessarily be affiliated with the originating website or application.

But, this might not tell you a lot(me either) so I'll give an example.

Real life example

Let's say you want to know about something that happens at a GitHub repo, a Docker Hub project or maybe you want to listen to event from you logs service? With Webhook you can accomplish that. Basically, A Webhook is an HTTP request, with a data(payload) or without, which sent to your server from external server.