The Event Stream tab shows the most recent notifications being sent to your users. It has live updates as notifications are sent to your users. You can see the notifications on the left side. If you click on one of the notifications in the list, it will show details on the right side. There are 2 sections: Raw Data and a Preview. The Raw Data section shows the data that was received by the Ravenhub API and the Preview section below shows what the user will see in their notification center.
You can use the event stream to debug issues with templates. For example, if notifications aren't appearing properly, you can check that the data is coming through in the Raw Data section and check that against the template to make sure that variable names are properly matched. It also allows you to see what your users are going to see without logging into each of your user's apps to see for yourself.