ROKU is a streaming box/stick.
You can download "channels" to it and stream stuff. (Netflix, Hulu etc)
I love my Roku(s). In addition to the 'premium' stuff (Netflix, Hulu, Shudder, and so on), it also has some good free channels (including Pluto TV and the Shout Factory channel). Best of all, it's just a hardware purchase -- get a Roku box for between $50-$120, depending on what generation you opt for (I have a couple of the $50 Roku 1s, and am happy with them), and you don't pay anything monthly to Roku itself for the service. I also found it pretty easy to set up.
Going through the comments on the reddit page, I see torgospizza is answering a lot of questions on Rifftrax Prime. It looks like it will be available through Roku (among other services), and it looks like streaming titles you already own and have in your library will be streamable through it, but it's not clear on if there's an option for just doing that without also paying the subscription.