TDEE stands for Total Daily Energy Expenditure and this calculator is used to determine how much calories you spend on a daily basis. TDEE is the sum of resting metabolic rate (RMR) and daily activities.
RMR or resting metabolic rate is the amount of energy that the body spends in order to perform basic bodily functions like blood circulation and breathing. Even when we are resting (sleeping for example) our body needs energy and burns calories. On the other hand, daily activities include walking, cooking, exercise etc. The more active you are, the more calories you burn.
If your goal is to lose weight, then you need to create a calorie deficit. In other words you need to consume less calories than TDEE. A good recommendation is about 500 Cal deficit a day which should lead to loss of 1 pound a week (every 3500 calories you burn equals 1lb). If you want to gain weight you should consume more calories than TDEE. And in case you want to maintain weight, consume equal calories to TDEE.