The calculator gives you an "unadjusted RMR" figure and a "sedentary RMR" figure which is the first figure multiplied by 1.2. This "sedentary RMR" figure already includes the Thermic Effect of Food (so yes, it assumes you are eating, it would not apply if you were fasting).
Can you just add the exercise calories? The answer yes Yes, but there is another minor "gotcha" which is not to forget that you were already burning calories during your exercise time.
For example let's say that your unadjusted RMR is 1400 so your sedentary RMR is 1680. You walk on a treadmill for one hour and it says you burnt 400 calories. Can you simply do 1680+400 = 2080 ?
Well what would have happened if you spent an hour not exercising? 1680 / 24 = 70. You would have burnt 70 calories anyhow. So the slightly more accurate calculation would be:
Yes, that does make sense. Thank you. One other question: when they say do not eat below your bmr rate does that mean that you shouldn't net below that rate? If my bmr is 1260 ( I'm a small person) and I burn 400 calories should I not eat below 1260 or 1660 to maintain?
Well that question and indeed the whole concept of "net calories" is a bit confusing, but let me provide you with some relevant information.
What matters for weight loss is the calorie deficit. This can be created by eating less or exercising more (or both).
You say you are a small person - well that can be quite relevant - what specifically matters is how much fat you could stand to lose (for example how many pounds of fat separate you from being 'underweight')
Maximum fat mobilisation is 31 calories per pound of fat. Someone who is very lean and only has 5 pounds to lose can only have a deficit of 155 calories a day. So that person will be eating back almost all of their exercise calories. So this 'net calorie' business may be appropriate to them.
On the other hand someone with (for example) 40 pounds to lose will not be eating back their exercise calories. They will try to exercise as much and eat as little as they can. Their maximal deficit is 31 x 40 = 1240 but in practice they may struggle to create it.
So your next job is to figure out how many pounds of fat you have spare (this is not necessarily how many pounds separate you from goal weight - because your goal might not be to be underweight)