Macronutrients help your body’s physiological processes. You have carbohydrates, fats, and proteins. To make sure that your body is in top shape, you need a well balanced food intake and these three categories are your prime concerns.
Carbs will provide energy to your cells and the fibre-rich type is important for your digestive system. Proteins are really the major pillar for building muscle but also to maintain muscle mass. Then we have fat, which at a very low level keeps your cell membranes in tip top shape, as well as absorbing vitamins A, D, E, and K.
You should focus on the balance of these three categories. First of all, the ideal ratio of macronutrients is not the same for all people. But in general about half of your macronutrient intake should be from carbohydrates, one third fat and just over a tenth protein. This changes when you work out frequently. Especially for body builders the ratio of protein intake has to be increased. At the same time, if you want to loose weight, decreasing your carbohydrates is an efficient method but it does has its pitfalls. The less fibre-rich food, can harm your digestive system as well as cause hormonal imbalances.
Essentially you should adapt your macronutrient intake to the type of fitness regime you have at the time. What you eat should definitely go hand in hand with how you exercise, and for what fitness goal.
For a very thorough overview of the macronutrient ratios and the math to use when calculating the macronutrient ratio, check out this article by bodybuilding.com.