I don't know if they have changed this in the few years since I let my Duolingo streak lapse, but they actually have decent explanatory text for each lesson, it

I don't know if they have changed this in the few years since I let my Duolingo streak lapse, but they actually have decent explanatory text for each lesson, it's just not (or wasn't) accessible from the mobile app; you had to use the desktop website to see it.