You lose a heart every mistake you make in lessons, to regain them you do a practice session, or wait and they regen over time... like health in a videogame.

I'm guessing you pay for duolingo? Or maybe it is different on smartphones, idk but it has had hearts for a while.

This sounds way better, it really is annoying to fail your daily lesson and then have to go do practice to keep your streak intact. I regularly fail grammatical gender in danish and can't bother trying any harder so this is nice.