I'm sure that you've already used metadata tags in your flex applications. There are several reserved keywords like [Bindable], [Embed], [Event] and so on (the full list is available here). The function of the metadata tags is to provide additional information to the flex compiler. For example information about embeding assets, dispatching events. This article doesn't explain how to use the default flex tags (you can learn that from here). It shows you how to create your own metadata tags and how to use them.