Top: drum 壴 (twig 十 over a bean 豆 [here: image of a drum 豆 on a stand]), bottom: 'to add' 加 (For getting power 力 your mouth 口 needs sth. to be 'added' [=to eat])
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!
