Macronutrients are those nutrients that provide energy for growth and to maintain metabolism.
They are divided in carbohydrates, proteins and fat.

Carbohydrates consists in carbon, hydrogen and oxygen atoms, according to the empirical formula (CH2O)n.
They comes from fruits, cereals, vegetables and tree (leaves, bark…). Plants are able to produce them combining water and carbon through chlorophyll photosynthesis.
According to the classical partition, they start from singular groups of particles, called monosaccharides (eg. fructose), that, combining each other, form disaccharides, oligosaccharides, and polysaccharides.
Usually carbohydrates are stored under sugar, starch of fiber form. The latter is not digestible by digestive enzymes but it’s very useful for maintaining a healthy digestive system by easing defecation and lowering the serum cholesterol.
They are stored in the liver and in the muscles in the form of glycogen and they are primarly a fuel for the human body, then a protein sparer (without them, the body will start to producee energy burning lean tissues), the main source of brain fuel and the main substrate of fat oxidation (without CHO, body will start to burn more fat, in a “dirty” way).
Liver is the main “tank” of glycogen, and, as example, after 1h of high intensity exercise, about 55% of stocks will be burned.
A sedentary person should have an intake of 40-50% of CHO per day in calories, amount to adjust at 60-70% for highly trained athletes.
Fiber should be 20-40g a day