Food should be healthy, that is it should have all the nutrients needed for the body. Like, carbohydrates, Proteins, Vitamins and Minerals, etc. A balanced diet is that  gives your body the nutrition it needs to function properly. 
A balanced diet is important because your body's organs and tissues need proper nutrition to work efficiently. Without good nutrition your body is more prone to diseases. 
To achieve a balanced diet we should include fruits, vegetables, grains,  meat / bean , dairy products, oils, etc.