Webhooks are APIs, but backwards - they’re ways for applications to send data to other applications, automatically. Instead of APIs - which are passive - webhooks are active, which means you don’t need to constantly poll them for updates. Popular business apps like Stripe and Github provide webhooks for things like customer checkout or pull requests.
The most popular use case for webhooks is small integrations, or getting your business apps to talk to each other in the ways that you need them to.