Your RMR (Resting Metabolic Rate) is the amount of calories your body burns in 24 hours at rest e.g. maintaining vital brain and organ function. Lets say that your RMR is 1500.
Naturally throughout the day you will burn more calories through physical activity (NEAT) and exercise (EAT). So lets say you burn an extra 500 calories per day. This now takes your Total Daily Energy Expenditure (TDEE) for the day to 2000 calories
Now we know that to lose weight we need to be in a calorie deficit (using more than you are consuming). Therefore, we can use our RMR as a guide to how many calories we should be eating per day.
Using the previous example, if you start to consume 1500 (RMR) calories per day but are using 2000 (TDEE) calories per day then you will be in a 500 calorie deficit and thus will lose weight.
However, its important to note that you do not want to consume less than your RMR as this may lead to a loss in lean muscle mass.