Action filters can be used to annotate action methods in ASP.net - that's not new. But in ASP.NET Core the filters can participate much more fully in the dependency injection story. You no longer need to jump though container specific hoops or make use of a service locator pattern. As an added bonus in this episode James throws Simon under the bus and makes him write GenFu code on the fly.
Read all about filters in the official documentation at https://docs.asp.net/en/latest/mvc/controllers/filters.html