A webhook is basically the way a GitHub repository "talks" to a web server whenever the repository is pushed to.
As the webhook guide explains - These “webhooks” can be used to update an external issue tracker, trigger CI builds, update a backup mirror, or even deploy to your production server.
Webhooks can now be configured directly as part of the repository settings (no API needed!). And webhooks are now easier to debug and even re-send. Webhooks are already one of the most widely adapted GitHub integrations and now that they are even simpler to use, will surely continue to be so!