Protein is one of the three macronutrients, alongside carbohydrates and fat. All macronutrients are important and must be consumed to thrive. Both the proteins we eat and the proteins in our bodies are made up of small compounds called amino acids. Amino acids are often called the "building blocks" of protein because when amino acids get assembled together, they form a protein. It may be helpful to think of amino acids as the cars on the train. Each car is an amino acid, yet the whole train is the protein. Amino acids are strung together in a variety of ways and combinations. The final sequence of amino acids determines the protein's function, structure, and role in the body.
While there are tons of amino acids in nature, humans need only 20 of them. Nine of these are called essential, meaning we must obtain them through diet. When we consume proteins through food, our body breaks them back down into amino acids, which is the functional structure for how they work in the body. How does it work? When we consume food, our digestive enzymes get to work in both the stomach and intestines. They break down those proteins back to amino acids, which can be reused to make the proteins the body needs.
In the body, older proteins are constantly being broken down while the new ones we consume are being synthesized. Some of the amino acids obtained through the diet can also be spared for energy if the carbohydrate intake is too low. In other words, if you consume too few carbohydrates, the body is starved for energy and will break into the tissues and convert the amino acids to usable energy.