First thing to make sure is that you have a paid plan, webhooks are only enabled for paid users.
Then click on a business > head into your settings in the top right of your business > settings > webhook tab > paste in your catch webhook > save > turn on webhook > create a test appointment to send a test pay load to your external system
This will send appointment data when an appointment is made, rescheduled or cancelled to your external system.
You can get your own system to log expenses and issue invoices, send emails, move trello cards and more. Below is an example of what you can do with the catch webhook from zapier.