Написать ответное письмо) You wrore me once that you had been…

Написать ответное письмо) You wrore me once that you had been learning English for about five years. I've started to learn it recently as a secondforeign laguage. So i think you are more competent in Enhlish that i am. Yesterday I came across the following sentence. "One apple day keeps doctors away". Could ypu explain me the meaning of it.