An apple a day keeps the doctor away is a commonly used proverb that actually can be considered in two ways.
1. An apple a day can keep you healthy.
2. Do your work regularly and you will face no problem.
It says that one should do his daily work regularly in order to be free from problems.
1. The old man who lives nearer to me drinks a cup of milk everyday in order to be away from sickness. He does not forget the proverb "an apple a day keeps the doctor away".
2. Arun who was very weak was asked to eat an apple every day to become strong. The doctor also advised him that an apple a day keeps the doctor away.