A Webhook is a unique URL that you can use as a developer for your apps. Some external services, e.g. a smart thermostat, report events by POST-ing to an external server. For example when the temperature has changed, because the user pressed a button on the device.
Of course, you want to keep that synchronised with your Homey app. But a webhook is usually static, and a Homey app can be anywhere. This service generates a static webhook URL, which will forward the incoming message to one of your Homey apps.