Here on Mastodon, we have a thing called a Content Warning, or a CW for short. It's a button at the bottom of the new toot box. When you hit it, it will add a new text box above the new toot box; whatever's in the main box will now be hidden behind whatever you type in the upper box, with a "SHOW MORE" button like this toot has.
We use this to hide anything we think will make our friends unhappy to read. So if you're posting about something that an American politician did that made you angry, you might give it a CW of "uspol". There's an additional subtlety that's evolved; a lot of the time we'll follow a CW with a +/- in parenthesis - "uspol (-)" would be bad news, "uspol (+)" would be good.
This lets Mastodon function as a place for more measured discussion; you can CHOOSE to engage with whatever bullshit the world is throwing at you this morning, instead of forcibly having your attempt to catch up with your friends interrupted by a million retweets of today's shitty news with a big photograph of that asshole who made that shitty news staring at you from the preview card. Not ready to deal with whatever bullshit's going on yet today? Don't click those "show more" buttons, keep scrolling, say something nice to your friends instead.
Different instances have different rules about what you MUST CW; the only requirement on my instance is politics. Check /about/more on your instance for their rules, see what they would like you to put this safety valve in front of.