It makes it so that if you visit another instance using their URL, it will redirect you to your home version’s federated link.
E.g. if your home server is Lemmy.world, let’s say you find a link to Lemmy on another website but it’s a link to Lemmy.ml. If you click the link, you won’t be signed in.
So like:
https://lemmy.ml/c/memes would redirect to https://lemmy.world/c/memes@lemmy.ml
This way you can seamlessly use Lemmy.
Though I’m not familiar with this particular script. Scripts should be verified for security before installation.
To be fair a lot of people have it on TestFlight because originally that was the only way to get it.