Reddit is a US-based social media site which is centered around topic-specific communities called "subreddits." Users can submit text or image posts to be voted on in each community, and users can also comment and vote on comments.
Fears of censorship of certain communities have led to users looking for alternatives. For example, the /r/GenZedong subreddit's fears of being censored has led them to join Lemmygrad, a pro-communist instance of the FOSS alternative to Reddit, Lemmy.