It isn't just part of what's on your plate or in your shaker bottle. It's a major part of who you are, and the same goes for every other living thing.
Proteins have existed on earth for billions of years and are the primary building blocks of most forms of life, from the simplest bacteria to the most sophisticated life forms.
In fact, it isn't really a thing, as much as it is a category of things. There are millions of different types of protein molecules, all containing different combinations of 20 standard amino acids. But proteins are also by far the most complicated molecules in the human body, and can contain anywhere from 50-2,000 amino acids including the essential 20. To date, about 10,000 different proteins have been identified in the human genome. There are thought to be nearly one trillion different variations on earth.
The body of a 154-pound man contains about 24 pounds of protein. Almost half is in the form of skeletal muscle, with another 3-4 pounds in the form of skin and blood. Bone and organs make up the rest.
In nutritional terms, protein is one of the three primary macronutrients our bodies need to survive. It is a "macro" nutrient because, like carbohydrates and fats, we consume a significant amount of it and use it for fuel. We need a comparatively small amount of vitamins and minerals every day, so they are referred to as "micronutrients."
Protein is contained in various combinations and quantities in every food we eat—even vegetables. Unlike all these other macro and micronutrients, your body can't store it, so you have to get it consistently through the foods you eat, and/or the supplements you take.