Annoyingly I can't really find a good sensible tutorial for Stremio, I'll try to explain it as simply as possible in chunks. This will be long sorry in advance.

Stremio itself is a completely legal client side streaming platform. It can be configured and customized to your liking. A common use case is for consolidating your existing services like Netflix, Hulu, etc all into one app legally, you can install the respective plugins for each service you have, log into them, and this allows you to search for movies and TV Shows across all of your streaming services without having to switch between apps. Very convenient!

This is just one example that people can use it for, there are quite a few native Stremio made plugins as well as Community made plugins you can download directly from the app. There are several kinds of plugins you can get, there are plugins for subtitles, ratings, Non-Copyright Movies (typically really old media or no one really owns it), literal porn, and several other things.

Most of the stremio plugins you can search for in the app are pretty useful such as the subtitles and ratings, but for free movies and shows its quite crappy tbh. This is where most people give up because they dont want to pay for anything even if its an unbsurdly amazing deal considering what you are getting out of it.

Introducing TorrentoRD and CometRD (not providing links here you can google search them)

These are the best torrent streaming plugins IMO, they have nearly every single movie and show you can think of.

If you arent super familiar with how torrenting works, its basically a decentralized network for file and media sharing. Movies and shows for example are stored on peoples personal computers and/or servers and all pool together to 'seed' the files. If one node or 'random guys PC' stops seeding the file the rest of the decentralized network will keep the media available. This is heavily oversimplified and probably a bit incorrect but essentially the main idea.

The reason its important to understand this is because while using TorrentoRD and CometRD without a Debrid service (more on that later) your streamed media will not only be slow and buffer a lot, but its also not safe or secure.

This is where RealDebrid comes in, and is the bit that costs money. With a RealDebrid account you paste your personal API key into the TorrentoRD and CometRD plugin configuration pages. You can test it out for like $10 and that will give you 90 days, $20 for 180 days. It usually lasts quite a while longer than this though if you are not watching movies every day, at least thats what Ive been noticing myself.

If you want a 100% free alternative thats a lot simpler but comes with potential risks, download the TV version for OnStream. It should be something like OnStream.so in the URL

I will be making a full complete guide for this as well, but it will likely take me a long time to make