This idiom is thought to have originated in Wales in the mid 1800s. It means that eating fruit keeps a person healthy. According to phrases.org.uk, it was originally penned in 1866 in Notes and QueriesMagazine

"A Pembrokeshire proverb. Eat an apple on going to bed, And you’ll keep the doctor from earning his bread.""A Pembrokeshire proverb. Eat an apple on going to bed, And you’ll keep the doctor from earning his bread."