TDEE (total daily energy expenditure) is the total amount of energy your body uses on a typical day. If you eat exactly that much, your weight stays stable. TDEE is therefore the same thing as your maintenance calories.
What it is made of
Your TDEE consists of your basal metabolic rate (BMR), meaning what you burn at complete rest for breathing, heart, brain and body temperature, plus everything on top: everyday movement (NEAT), training and digestion (the thermic effect of food). BMR is the biggest block; everyday movement and training vary the most from person to person.
How calculators estimate it
Most calculators first estimate your BMR, usually with the Mifflin-St Jeor equation, and then multiply it by an activity factor between 1.2 and 1.9.
Example: With a BMR of 1,800 kcal and a factor of 1.55 for three to five training sessions per week, you get a TDEE of 1,800 × 1.55 = 2,790 kcal. This is a worked example, not your number.
The result is an estimate. Even the BMR is off by more than 10 percent from the measured value for some people, and the activity factor is only a rough category. A range of about plus or minus 10 percent is therefore more honest than a single number.
From estimated to measured TDEE
Your real TDEE only shows up over time: eat roughly the estimated amount for two to three weeks and watch your trend weight. If it stays flat, the number fits. With Pro, Lean is law calculates an adaptive TDEE from your logged intake and the slope of your trend.