Remote Event Receivers in SharePoint 2013 allows you to respond to SharePoint events within an app like list added, list item added etc., Remote event receivers are only supported in autohosted and provider-hosted app. If you are new to remote event receivers, get a quick overview in here. They are basically WCF service endpoints to which SharePoint will respond when events occur in SharePoint. So, in other words when an event occurs in SharePoint and you have subscribed to that event, SharePoint will send the info along with event parameters to the remote WCF service.
However, debugging remote event receivers is a challenge as the remote event receiver service is hosted locally in your IIS Express environment and the SharePoint is in a remote environment.
I just published a blog post in the Apps for Office and SharePoint Blog covering how to debug remote event receivers using Windows Azure Service Bus Relay.
If you are using remote event receivers in your SharePoint App, then I would highly recommend going through the blog post which will help you debug remote events right from your local development environment 🙂
Feel free to comment here if you have any doubts or if you are stuck debugging remote event receivers.