Webhooks are a system for how one server can notify another server when something happens. Some sites, including GitHub, allow us to provide them with a URL, for instance, https://api.graphql.guide/github-hook, to which they make an HTTP request when a certain event occurs. If we tell GitHub we want to know about the watch event on the Guide repo, then they will send a POST to our server (using the given URL) whenever the repo is starred. The POST will contain a JSON body with information about the event, for example:
Then our server parses the JSON to figure out what happened. In this case, the sender is the user who performed the action, and we see under the repository attribute that the repo now has 9 watchers.