Typically, when you click on a link to a post on Lemmy or Mastodon (or any other Fediverse application), it goes to the instance that was linked. For example, I’m reading this on lemmy.world. If I were to link you to this post, it would load on Lemmy.world’s site. Since you’re reading the post on Lemmy.world instead of your account’s instance (lemm.ee in this case), you can’t interact with the post since you’re not logged into Lemmy.world. This add-on would redirect you from Lemmy.world to the same post on lemm.ee.

(TL;DR) Adding this add-on would redirect you from Lemmy.world (remote instance) to the same post on Lemm.ee (your account’s instance), which would allow you to interact with the post without having to find it on your account’s instance manually.