What is Webhook?
Web hook is a general concept for http based events, which will provide remote event mechanisms.
Say for example, you have a built a custom application separately which has some specific product related content. Whenever there is a data change on the product repository, your data on the custom application should also be updated. For this scenario, will you be keep on checking the product repository for the changes? The answer should be No. Instead of your application keep on poll the product for changes, product should notify or push the changes to your application.
What is SharePoint Webhook?
Previously for working with event based processes, we were using the server side event receivers and remote events receivers. Such processes can be replaced with the webhooks.