Aren’t they just used to refil your hearts when you have used them all up for a day?

I don’t know what language you are learning as I believe it is different depending on language but for Spanish Duolingo is honestly trash, if you actually want to learn a language rather than just having shit thrown and you and seeing what sticks I really recommend Busuu. For a start it doesn’t have a debilitating heart system that stops you from actually learning but it also explains concepts properly.