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