Janus was created to allow for people to connect to Minecraft servers using their Twitch accounts to verify their identity. It allows server owners to whitelist, decided who can join, their Minecraft server using Twitch usernames rather than Minecraft accounts. This allows them to do far more user control than normal.
For instance with Janus you can limit who can join by requiring them to have a connected Twitch account, you can require them to be subscribed to a certain channel, and using StreamElements you can require them to have a certain amount of watchtime in a channel. This allows Twitch streamers and server owners to make Sub, Viewer, or Twitch based SMPs much easier than ever before.
You can get started using Janus at janus.tgb.gg