TDEE stands for Total Daily Energy Expenditure and it is a calculation that tells you how many calories you burn per day based on your activity level, age and weight. Calculating your TDEE will help you understand how many calories you need to be eating in order to maintain, lose, or gain weight.
To train, whether it be strength training, weightlifting or cardio, you need physical energy. Knowing your TDEE against the training goals you have will help to keep your nutritional needs balanced.
If your goal is to gain lean muscle mass target approximately 500 calories more than your TDEE, which is enough to put on mass without adding body fat as long as the food you eat is nutritious and balanced. If your goal is to lose weight you can remove 500 calories from your TDEE.