In more detail
TDEE adds three parts: basal metabolic rate, the energy of digesting food (roughly 10% of intake) and all physical activity, from exercise to fidgeting. In practice it is estimated by multiplying BMR (basal metabolic rate) by an activity factor: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active and 1.9 extra active. Wearables estimate it from heart rate and movement, with errors that can run to several hundred kilocalories.
Why it matters
TDEE is the reference point for weight goals: eating below it loses weight and above it gains weight. A deficit of 500 kcal a day is about 3,500 kcal a week, roughly 0.45 kg (1 lb) by the classic rule, though loss usually slows as the body adapts. People often overestimate their activity level, so treat the figure as a starting point, track weight for two or three weeks and adjust. Large deficits, pregnancy or medical conditions call for professional advice.
Example
BMR 1,780 kcal (man, 30, 80 kg, 180 cm)
Sedentary × 1.2 = 2,136 kcal
Lightly active × 1.375 = 2,448 kcal
Moderate × 1.55 = 2,759 kcal
Very active × 1.725 = 3,071 kcal
Extra active × 1.9 = 3,382 kcal