@evilmicrowizard it's not an instance or mod construct, it's core to Mastodon culture. It's one of the best features of Mastodon over Twitter.
You can Choose what you feel like seeing that day, as opposed to some algorithm pushing stuff at you.
From the mastodon onboarding guide:
One of the best features on Mastodon is that button that says āCWā where you write your toots. Clicking that adds a content warning field where you can put information about what the toot contains (eg. mental health, politics, lewd talk, nudity) so that people donāt have to see content they wish to avoid, for whatever reason. Of course, itās also great for show or book spoilers.
A common convention is to put +, -, or ~ in a content warning to signify if the contents are broadly positive, negative, or mixed respectively.
My advice is simple: if youāre not sure whether a toot needs a CW or not, give it a CW. People really appreciate it and it doesnāt do any harm to be too cautious and too respectful of others.