Total Daily Energy Expenditure — your maintenance calories, from BMR and activity level.
It uses the Mifflin-St Jeor equation: BMR = 10×weight(kg) + 6.25×height(cm) − 5×age + 5 (men) or −161 (women). That's your estimated energy use at complete rest.
BMR is multiplied by a factor for how active you are day-to-day: 1.2 (sedentary), 1.375 (light exercise), 1.55 (moderate), 1.725 (active), or 1.9 (very active/hard daily exercise) — giving your estimated maintenance calories (TDEE).
It recalculates TDEE using your same BMR but each of the other activity multipliers, so you can see how your maintenance calories would change if you were more or less active.