Fat is a macronutrient made of triglycerides (esters of glycerol and fatty acids) that carries flavor, tenderizes, and enables browning in cooking. It comes from both animal sources (butter, lard, tallow) and plant sources (olive, coconut, avocado, and nut oils), each with its own flavor and smoke point.
Fat matters for technique as much as taste: it conducts heat evenly in sautéing and frying, creates flaky layers in pastry, and carries the fat-soluble vitamins A, D, E, and K. The right fat for a job depends largely on smoke point: refined oils with a high smoke point suit high-heat searing and frying, while butter and extra-virgin olive oil, which smoke at lower temperatures, work better for gentler cooking and finishing.