Structural proteins in foods, i.e. albumin in eggs, myosin in muscle meats, gluten in wheat, are formed by amino acids in complex structures. Proteins are folded and clumped chains of peptides, and peptides are chains of amino acids. Your body uses stomach acid and proteases (enzymes) to break down proteins in digestion, but this requires time and energy (stomach churning, body heat).
The amino acids are what your body requires. They're used to build more complex proteins. Cooking gives a head start in un-clumping/unfolding proteins, and in some conditions into peptides and amino acids, that are easier for your body to digest making them more 'bioavailable'.
Some of these amino acids can be produced by your body using nitrogen compounds from breaking down other amino acids and nutrients. The ones your body can't produce on its own have to be obtained in the proteins you eat. These are called 'essential amino acids'.
Different protein sources have different amino acid compositions, giving them different 'protein values' in how complete the range of essential amino acids are available; i.e. collagen/gelatin is a common structural protein that provides great texture for sensory applications, but the protein value is 0 for regulatory labelling requirements in some countries since it is missing an essential amino acid.