The idea behind Channel Progression is that viewers can be rewarded based on their engagement with a stream. With most streaming services, you're either a subscriber or you're not, so there's no way for the streamer to know who they're broadcasting to.
With Channel Progression, viewers are ranking up based on watching, chatting, following, and subscribing to channels. The feature will be available to Mixer Partners at first, where they will be able to offer things like gift subscriptions and Embers spend (Mixer's real money-based currency).
Streamers could also tailor their content based on who is watching. Since the streamer can see the levels of who is watching, they might want to tailor the content to newcomers to their channel, or to more experienced subscribers.
An early version of Channel Progression will be available on the desktop platform beginning tomorrow, and it will be coming to the mobile apps in the coming weeks.