What's a subreddit? A forum dedicated to a particular subject on Reddit. Subreddits have their own rules and moderators to help maintain a sense of community within specific topic areas.
Why? Reddit says the groups were posting personal information, which violates Reddit terms of service. In a statement to Axios, Reddit says they're clear in their terms of service that posting of personal information can get users banned from Reddit and that they've banned r/altright and r/alternativeright "due to repeated violations of the terms of our content policy."
What we know: According to IBTimes, the r/altright group had 16,007 subscribers and increased in size by 37% around the time President Trump was inaugurated. The forum was created in 2010 but didn't gain traction until last summer.