Webhooks are a general term for when one system sends data to another system automatically. There can be webhooks for anything, such as "liking" a video on Youtube can send a webhook to save that video to a spreadsheet, or purchasing something with PayPal will send a webhook to your bank account to deduct money.
Fuchsia provides the ability to "hook" into some actions that happen in your system and do something else automatically. For example: user ABC saves a new Product to their project collection in Fuchsia. That user can register a webhook to send a notification to their website and mobile app to update the product catalog whenever a new product is added, or removed, or edited.
Some more examples of Fuchsia's webhook capabilities:
Send a notification to a Continuous Integration/ Continuous Deployment (Like CircleCI, TravisCI, Github Actions, Gitlab CI) system everytime some data changes so that their sites can be rebuilt with the new data.
Send a message to a custom server that can use that message to update a seperatly connected system.
Send a message to an email server to send your subscribers an email whenever a new article is published
Send a notification to Zapier, IFTTT, Jira, Github or any other service that accepts HTTP requests.
There are millions of applications that exist that can connect with webhooks, and any system that can - means that they can connect to Fuchsia for live changes.