As others have mentioned, it’s a way to link to a community. More specifically, it’s what allow to share a link to a community (not to a specific post, just the community itself) that will work no matter what instance anyone is clicking it from, unlike your usual link.

Specifically, [email protected] links to the local copy on your instance of /c/[email protected] and thus, allows you to post and comment using your account on your instance, whereas https://feddit.org/c/[email protected] links directly to feddit.org and would only allow you to post and comment if you have an account on feddit.org.