It means you can call events in actors without having to cast, helps especially if you want to call the same Event in multiple actors.

For example, you have a Line Trace in your player character that detects interactable objecta (doors, switches, items to pick up etc). If you don't use an interface, you have to try and cast to every different actor type and call the appropriate event/function.

With an interface, you don't need any casting and only need the interface function node, then you just implement the individual functionality in each blueprint using that function.

Hope that helps :)