If you're asking about
https://news.ycombinator.com/item?id=49937309, users flagged it. When a post gets a lot of flags relative to upvotes, HN's software kills the post (that's what "[dead]" means). Moderators sometimes also kill posts, but mostly they're killed by software.
Dead posts are removed from public view, but you can still see them by turning the 'showdead' setting on in your profile. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.
If you turn 'showdead' on, please don't forget that you did! As your last sentence implies, turning it on means signing up to see a ton of dreck. (https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...).
You'll also see posts that don't deserve to be [dead] and for that we have something called vouching - also in the FAQ at https://news.ycombinator.com/newsfaq.html ("What does [dead] mean?")