Total Daily Energy Expenditure adds your everyday movement and exercise on top of your resting burn. It's the single most useful number for weight management, since eating close to your TDEE keeps your weight roughly stable, while eating above or below it drives gain or loss.
TDEE multiplies your BMR by an activity multiplier: 1.2 for a sedentary lifestyle, 1.375 for light activity, 1.55 for moderate activity, 1.725 for a very active routine, and 1.9 for extremely active people such as those with physical jobs or two daily training sessions. It's an estimate, so real-world results can vary by a few hundred calories.