How does lemmy fit into the fediverse? - eviltoast

Sorry, I am really new to the idea of the fediverse. So, it is about standartisation for social media protocols so I can interact with users on another side that uses the same standard. Does this mean if I post something on lemmy I can find it on Mastodon, or is it only certain features? I guess I dont understand how this applies to the every day use of there application