Sorry... So basically Plex is a "channel" you can install on a Roku, an app on Chromecast, or an app on any iPhone or Android device.
But the above are useless if you don't have a Plex server running on the computer where all your media is stored. Let's be honest, the only reason this exists and is so popular isn't because we all have mass home videos we want to show to all our friends. No, we have TV shows, and movies, etc that we need to play on our TV. I'm sure many are like me, and I don't actually subscribe to any TV service.
Now the Plex server isn't exactly difficult to setup, but takes a little time. Might be a little over the head of Grandma and Grandpa, but it's not that difficult. You set it up, tell it where your TV shows are stored, tell it where movies, music, etc are stored, and you are off and running. I have it set to update and rescan whenever it detects changes. So with my setup, whenever I get a new TV show or movie it's updating, so that I can always watch whatever I just got.
Now, forgive me if this is a little wrong, as I've not heard anyone really talk about the step of needing to setup the Plex server on the PC first. As far as I know this is still required if you want Plex to work. After that the Plex channel or App will work great!