I’ll give it a try, though I’m new here too:

What’s an instance? -> A server on which you can register your account

What’s a community? -> same as a subreddit

What are federations? -> Information exchange between servers

What’s mastodon? -> Twitter alternative. As lemmy is to reddit

What’s Kbin? -> Similar to lemmy, but kbin can interact better with Mastodon

What’s ActivityPub? -> the protocol used within the fediverse