Protein is made of amino acids and is essential for building muscle, repairing tissue, producing enzymes and hormones, and supporting immune function.

Essential vs. non-essential amino acids:

• Essential — 9 amino acids the body cannot make. Must come from food.
• Non-essential — 11 amino acids the body can synthesize.

Complete vs. incomplete protein:

• Complete: Contains all 9 essential amino acids. Found in meat, fish, eggs, dairy, soy, quinoa.
• Incomplete: Missing one or more essential amino acids. Most plant proteins.

How much protein do you need?

• Sedentary adults: 0.8g per kg of body weight
• Active adults: 1.2–2.0g per kg
• Building muscle: Up to 2.2g per kg

Reference:

Wikipedia: Protein (nutrient)

https://en.wikipedia.org/wiki/Protein_(nutrient)