A truncation of the proverb "an apple a day keeps the doctor away," meaning small, healthful everyday habits will help prevent medical problems in the future. People consider me eccentric for my insistence on going for a walk at dawn each morning, but an apple a day, as they say!
A small preventive treatment wards off serious problems, as in He exercises regularly-an apple a day is his motto. This idiom shortens the proverb An apple a day keeps the doctor away, first cited about 1630.