The saying an apple a day will keep the doctor away has been around since the 1800's.
Apples are very healthy for you and have vitamins and nutrients that are essential to optimal health and by eating an apple a day your health should improve which means that the more healthy you are the less often you'll need to see a doctor.
Still it's a good idea to see a doctor on a regular basis for a health checkup so in reality just eating an apple a day won't keep the doctor away.
But eating apples helps your health improve so that you won't be as ill as some people would be if they didn't eat apples or other healthy fruits.
You should make apples and other fruits a daily part of your meals to keep your body healthy since apples and other fruits can help give you energy and boost your immune system and may actually prevent certain diseases including cancer.