A phone call from another service to your app when an event happens, so you can react right away.
Like a text message from a courier saying a package was delivered. A webhook is a message another service sends to a URL you control when something happens.
It lets your app react right away to payments, signups, or other events, instead of checking over and over. Stripe, GitHub, and many others send webhooks.
You create an endpoint, a web address that runs code, verify a shared secret token to be sure the message is real, then do work, such as updating Supabase or starting a job in Modal.