Instead of making an account on one site (reddit) and only being able to use that account on the one site, you can create an account on one instance, and then participate on every other instance that is federated with your instance. Logging in using one account between otherwise isolated websites is federation.
Instances lemmy.world lemmy.ml lemmy.sdf.org Here’s a huge list of instances: https://lemmy.fediverse.observer/list
Instead of making an account on one site (reddit) and only being able to use that account on the one site, you can create an account on one instance, and then participate on every other instance that is federated with your instance. Logging in using one account between otherwise isolated websites is federation.
Here is what a link to another instance looks like: https://lemmy.world/c/fediverse@kbin.social
Logged into lemmy.word, but we can browse and participate in communities (fediverse) on other instances (kbin.social).
Another example: https://lemmy.sdf.org/c/piracy@lemmy.dbzer0.com
Logged into lemmy.sdf.org, browsing the piracy community on the lemmy.dbzer0.com instance.
And a good write up on the fediverse: https://blueskyweb.xyz/blog/6-23-2023-moderation-proposals