An apple is a round fruit with red or green skin and a whitish inside. One variety of apple might be sweet, another sour.
The apple isn't just a fruit. It's a symbol — from the apples eaten by Adam and Eve in the Biblical creation story, which symbolize the loss of innocent to the expression "American as apple pie" to mean something that is wholesome. The "apple of your eye" is the person who delights you — maybe you are the apple of your grandfather's eye. He might tell you, "An apple a day keeps the doctor away," meaning the fruit is good for your health.