A 3rd party app is anything you have added, such as any Antivirus that's not Inherent in Windows, real temp, CCleaner, Ms office, Adobe reader, Java updates, nvidia or AMD software, your specific printer software, router software, Asus suite, Norton, flash player, iTunes, logitech gaming software, razer mouse/keyboard drivers, Corsair Link.
Pretty much anything you've added or carried over from the prior OS that starts/runs automatically when windows starts, or something you personally are running prior to running a game. There have even been issues with Edge software, Bluetooth drivers, stuff in Windows 10 that applies to laptops or tablets and has no bearing on a pc like searching for wifi hotspots, switching between data/wifi etc.