Houdini has some tools that allow you to create things procedurally and you can then set up some parameters that allow you to change settings in the asset, the plugin basically brings that functionality to something like UE4, so you can create an asset in Houdini and then export that to UE4 and it will have those parameters available for you to adjust within Houdini. It’s like they took the core of Houdini and put it into a plugin so that you can make changes in a separate software and have it update without having to send it to Houdini to update changes.