Content warnings and sensitive media
Every post you write carries a Sensitive mark, whether or not it has a picture, video, or audio attached - words need a warning as often as pictures do.
On its own, checking it covers any attached media behind a cover the reader has to open before it shows; the words stay visible. Checking it on a post with nothing attached covers nothing, since there's no media to hide - if it's the writing itself that needs the warning, use the warning text field that appears once Sensitive is checked.
Add warning text and the effect changes: the whole post moves behind that warning instead of just the media - words, poll and all. The commonest warning of all is a spoiler, and what a spoiler spoils is usually the words.
Nobody sees a content warning's contents by default - it's a sentence written about this particular post, and there's no standing answer to a post nobody's read yet. Sensitive media is different: the Sensitive Media section of User Settings can show it without opening the cover, for anyone who'd rather it just did.
Across the Fediverse
Both travel in both directions: your covers and warnings show the same way to your followers elsewhere, and a post arriving from another server marked the same way is covered here too.