We know this because Chance The Rapper tweeted the words "Made an album with Lil B" on Saturday. Lil B followed it up with the slightly more specific "Just knocked out a based freestyle album with chance the rapper in chi town!!"
Since dropping his 'Acid Rap' mixtape in 2013, Chance hasn't taken the typical route of signing to a major and putting out his artist debut. Instead, he's stayed in the background, playing a supporting role in groups like The Social Experiment and generally just following the creative projects that inspire him. You can add this collaboration to that list of projects.
That's about all there is for specifics. We're sure this will be very, very good fun though.