The way to think about Duolingo is like another mobile phone game. Except the "universe" of the game is language learning instead of a random farming or middle ages theme. And so you learn about the language in the same way you learn about farming or middle ages from those games. Meaning if you never read anything about farming or the middle ages now you know of some things, but you'd really be much faster ahead by picking up a proper book on the subject. So it is with language learning apps.