Basically it works much like Reddit but there is no central server. It's a collection of servers belonging to individuals. Your account lives on one of these individual servers, but you can view and post on other servers as well. This has the benefit of having no central authority to dictate what everyone should do, but the downside of being more fractured. For example, if you are into video games, server A might have a gaming community, and server B might have their own separate gaming community. Nothing is stopping you from subscribing to both, though.
At the top you will have filters, Subscribed (the communities you are subbed to), Local (the communities on the server where your account is), and All (all communities across the servers that are federated with yours). I've been filtering by "All" and subscribing to the communities (subreddits) that I like, whether they are on my server or other servers. I've only been here since yesterday and I've joined ~25 communities so far.
Traffic is light at the moment and I've often filtered by "New" to see the newer posts. I plan on doing some posting today to contribute. Time will tell if the Reddit blackout and people quitting Reddit will increase the traffic. I imagine they would all call the same API, but I'm not a coder.