A webhook lets an app pass information to other apps in near real-time. This could mean notifying a private application developed just for you, or a way to notify another 3rd party application that accepts POST requests.
In Emercury, webhooks are used to send data from your account into another system or database that has an API (application programming interface) that can listen for webhooks (eg, Zapier). They are event driven and allow you to update 3rd party applications with contact and campaign activity as they happen.
You may want to use webhooks to keep subscriber records in sync with an external database or application, maintain an external unsubscribe or bounce list, or capture campaign opens, clicks, forwards, and shares to push to another application for your own custom reporting needs.