Essentially, Black Twitter is its own sub-section of Twitter that became prevalent around 2013 as a higher amount of non-white Americans online used Twitter. A lot of tweets involve humour relating to African-American Culture and commentary on systemic issues. The Wikipedia for Black Twitter has a decent overview of how it developed and its impact on social media. The subreddit was then made as a hub to share the funny, relatable, or insightful tweets from that section of Twitter. I hope that is a useful answer, I am a white Australian so my summary is based on what other people closer to that community have said.