Can anyone Explain how The Federation works to me like I am five years old?

Forum: People set up communities on their or someone else’s server. People have to individually find and register for each siloed community.

Aggregator site (i.e. reddit): Everyone sets up communities on the same server. A single account on the server can join and interact with any community they want, and aggregate them all together into a feed.

Federated aggregator (i.e. Lemmy): People set up communities on their or someone else’s server. They all use the same language so they can communicate with each other. You can make an account on any server and then visit any community, and aggregate them all together into a feed.