Making too many mistakes will cost you hearts, which can put your practice on hold for a few hours. While you can obtain more hearts by completing a review or watching an ad, upgrading to Super Duolingo will grant you unlimited hearts and remove any ads. It also lets you deepen learning in the Practice Hub (the dumbbell on your home screen), where you can work on weak areas with Target Practice, engage in conversation skills, review recent mistakes, and practice further via a library of interactive stories (available for select languages).