What is protein?
Protein is around 5kcals/g (see section what is a calorie) and consists of 20 different amino acids all of which play a different part inside the body. Some proteins will be responsible for cell regeneration while others will have mechanical functions used for muscle contractions (myosin & actin). Human beings only contain some of the 20 amino acids so the remaining few need to be extracted from the diet and then bonded on to other amino acids that are already present inside the body to aid metabolism and basically complete the set.
As most of the amino acids required by humans are already present, the daily requirement for protein from food for a sedentary person is only around 1.2g per kg body weight per day and even in a very active person such as an athlete usually 2g per kg body weight per day is more than sufficient. In a situation of starvation protein could be used as an energy source and enables the body to break down its own stored proteins for fuel (see section low carb diets myth-busted).