Plex = video streaming service that you can run at home on your own devices and with your own media. More useful than playing the files themselves as it'll convert stuff on the fly if it detects your device can't handle the format.
This deal = 1 year's subscription to Plex Pass which gives you premium features, as for what they are and whether they're worth it, idk, I use the free version.

New to Plex? You tell it where your media is and it has a best-effort go at detecting what it is, mostly from titles. So I say "this folder contains movies" and it scans the files within. It doesn't work 100% (I had a 2 minute train.mp4 file which it detected as the movie Train, that's 1 hour 34 minutes) but it works 95% of the time so is worth having over not having. It maintains its own database so the only thing you might have to do is rename some music files if they're not picked up correctly.