How are apples and other type of fruit healthy?
Certain types of foods are considered more healthful than others. One of the main reasons is something called nutrient density of the food items. Our bodies burn calories to produce the energy we need to move, keep our body warm, think, sing, etc. A slice of white bread or a can of soda has nutrients in it but almost all of it is carbohydrates. These foods are called "energy dense". The amount of other nutrients we get from these types of foods is relatively small. These types of foods are not considered to be "nutrient dense" (also called micronutrient dense). Fruits have a higher ratio of nutrients to calories and are therefore considered more "nutrient dense".
Fruits and vegetables are our main sources of many vitamins and minerals (also known as micronutrients). Some micronutrients can be made within our body (synthesized) from other things we eat and drink, but many cannot and we need to eat or drink items that contain these micronutrients regularly. Our body doesn't require as much of these micronutrients as it does things like carbohydrates and proteins, but they are vital to our body's growth, repair, immune system and many other functions. When people say that apples and other fruits are "healthy" (more accurately "healthful"), they typically mean as part of our regular diet.
Although apples and other fruits are considered healthful, eating only apples would make us very ill and eventually kill us. We need to eat a variety of foods, as our regular diet, to be healthy.
A marathon runner, in the last mile of a 26 mile run may find that a snack or drink high in sugar and salt would be the most healthful thing to eat or drink, at that time, because that is what his or her body needs at that time. For average daily living, consuming fruits and vegetables regularly is part of a healthful diet.
I hope this answers your question.