This functionality requires the application of AEM 6.3 Service Pack 3 (126.96.36.199).
Associated content provides the connection so that assets (i.e.) can be (optionally) used with the fragment when it is added to a content page. This provides flexibility by providing a range of assets to access when using the content fragment on a page, while also helping to reduce the time required to search for the appropriate asset.
There are various methods of adding visual assets (e.g. images) to the fragment and/or page.
To make the association you first need to add your media asset(s) to a collection. Once that is done you can: