What's Mastodon? Mastodon is basically Twitter, except with less trolls and Nazis. And longer tweets.
It’s also Twitter without a whole bunch of VC investment that it needs to try and pay back by monetizing your eyeballs and keeping you on the site for as long as possible, regardless of whether that's by making you happy or enraging you.
And finally, the last big difference is that instead of living on a bunch of computers run by people working for Twitter, Mastodon lives on a bunch of computers run by whoever feels like running an “instance”.
Each instance can set its own policies. If you really want to be a jerk to people, you can find or start an instance that’ll let you! But people on other instances can very easily filter out not just you, but your entire instance. The hope is that this will keep things more civil than what Twitter has turned into. The reality is that there are probably exciting new failure modes somewhere in the future.
The policies of this particular instance can be found here. The TL;DR version of them is "Imagine you have a cool queer aunt. Don't do anything that'd probably piss her off."