You see where there was a little "SHOW MORE" button just there that you pressed to see the rest of this? That's a content warning (or CW)

You use those whenever you want to make a toot (stop giggling at the back, we know, that's what it's called, get over it) that mentions something that other people might not want to see

Stuff like politics (it's exhausting), drugs, NSFW stuff, health discussions, selfies, and so on

That's a non-exhaustive list, just be considerate