Webhooks use an event-based mechanism, where the source system (in this case, Jira) will make an HTTP POST call to all the registered webhooks when a registered event occurs. This is very similar to Jira's internal notification system where emails are sent based on events.
With the event-based approach, instead of requiring the remote application to constantly poll Jira for changes, which is both inefficient and inadequate for situations where changes need to be processed in real time, the remote application can be registered in Jira with a webhook, and Jira will call the application when the event occurs.