Here’s an example. Say you want to visit the Engineering community on sh.itjust.works. The community lives at this URL:
https://sh.itjust.works/c/engineering

However, you are using an account at lemm.ee. In order to post or comment you need to access the community via lemm.ee, which means using this URL:
https://lemm.ee/c/engineering@sh.itjust.works

In the first case you are accessing the community directly. In the second case you are accessing the community through your local instance.