Federation means you can search for your community on your instance and it will fetch all the information if you want to subscribe. I am writing this comment with my account on feddit.de. I just switched to the „all“ feed, which shows all content from all servers feddit federated with. If I am looking up a new community to join like communityname@servername.tdl feddit will copy the posts for mento be able to interact with them. My reactions will be synched back.