Explanation
A webhook is basically a callback URL that is triggered when certain events occur, such as a code commit. If the webhook call from the code repository does not reach the CI server, then the pipeline will not start.
According to the Cisco Implementing DevOps Solutions and Practices using Cisco Platforms (DEVOPS) Study Manual, "A webhook is a web-based feature that enables a remote service to communicate with a local service, usually through a web server. Webhooks are used to trigger an action when a specific event occurs, usually in a remote service or application."