In many languages (such as French), short strokes are used above the letters to express an emphasis. This stroke 丶 is used in the mnemonics accordingly.
Vocabulary
主
ぬし
owner, master, lover, god
216
主に
おもに
mainly, primarily
217
主人
しゅじん
master, head (of a household), landlord, ones husband, employer, host
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!