Yes apples are a good food to eat for breakfast.
It's best to eat more than just an apple for breakfast and eat some cereal, toast, sausage etc to fill your stomach but even eating just an apple is better than no breakfast at all.
Breakfast basically means break fast where you are breaking the fast that you were in overnight when you're asleep.
When we are sleeping we are always without thinking about it fasting and in the morning we should eat something to break that fast.
You'll feel more energized and ready for the day when you eat something instead of going without breakfast.
I used to go without breakfast a lot and noticed I had a decrease in energy but after eating breakfast I'm more energized and get more done instead of waiting until lunch time.