It's basically a feature that shows comments from other communities, when there's a crosspost.

So say, when there's a link to an article on www.omgubuntu.co.uk and it's posted to https://lemmy.ml/c/linux, https://programming.dev/c/linux and https://lemmy.linuxuserspace.show/c/news, you can see comments from all three communities instead of having to check each of them separately to get the whole discussion.

(I think it only shows crossposts from federated instances. Or maybe only from the communities you have subscribed to? Not sure)