See here and here. It's software that runs on a PC, Mac, linux machine, or NAS that serves your media to numerous devices including phones, tablets, PCs, and the Roku, in and outside your network. It's an amazing piece of software IMO but typically requires a powerful machine depending on the kind of media you have. Again, I haven't tested ISOs, but I heard they work on the Roku. Otherwise, Plex generally doesn't support ISOs. MKV's are much better.