A Webhook is a callback to an application or web service that is triggered when a specific event occurs. That means you can set up a Webhook to look for a specific update, change or action to occur in your Helpdesk and it will automatically push the information you specify to the application you want. In simple words, two applications communicate using Webhooks.
Webhooks can be triggered via the automation rules that run on ticket creation and rules that run on ticket updates in Freshdesk.