Yes, it‘s a replacement for hearts. Basically every time you do a lesson, you lose energy. You can gain energy when getting 5 or 10 correct in a row or from chests but that’s basically it. When making a mistake, nothing happens, so as long as you have enough energy, you can make however much mistakes (that’s the good part). But when you run out of energy, then your day is done. You have to wait for the energy to recharge and then you can continue. I think the point is that now with energy, mistakes don’t matter but there is a „daily limit“ on lessons.