Controversial Reddit communities are places on the website Reddit where people talk about topics that some people might disagree with or find offensive. People on Reddit can join communities that have different opinions on different topics, and in some cases, these opinions can be seen as "controversial," meaning many people find them wrong or upsetting. People in these communities share their ideas and opinions with others, and sometimes things can get heated and people can argue. Some people might think these communities are wrong since they talk about things that might be seen as offensive, and that's why they are considered controversial.