Explain how peoper nutrition is connected to the main function of the digestive

I suppose you mean proper nutrition. Main function of digestive system is to break down food and turn it in to energy. You have most probably heard about calories. So, 1 gram of Protein is 4 kcal, 1 gram of Carbohydrates is 4 kcal and 1 gram of Fat is 9 kcal. (also let's not forget that 1 gram of Alcohol is 7 kcal) Average male needs about 2000 to 2500 kcal a day to function properly (it's slightly less for females it depends on bodyweight, amount of muscle mass and activity level) 

Along with water those are Macronutrients (carbs,protein and fat).
 
Micronutrients are vitamins and minerals essential for your body. For example Vitamin C, Iron, Zinc...

Now, proper nutrition is balanced diet filled with vitamins, minerals, quality sources of protein, carbs, fats and also filled with right amount of fiber which plays a vital role in your digestion by helping you with bowel movements and even lowering  your blood cholesterol levels. So basically proper nutrition is connected to main function of digestive system in a way that by supplying your body with everything it needs, giving you right amount of energy it also keeps your whole body healthy at the same time.