Public means a post can appear pretty much anywhere on the Fediverse and be seen by anyone.
Unlisted is the same as Public, BUT it won't appear on Explore and it won't appear on the Local or Federated timelines.
There's an explanation of Local and Federated here: https://fedi.tips/what-are-the-local-federated-home-timelines-how-do-i-stop-them-scrolling-too-quickly/
And here's an explanation of Explore: https://fedi.tips/where-are-the-trending-posts-and-hashtags-on-mastodon/
By the way, don't feel embarassed to ask, these are totally valid questions!
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