Net Calories?

Net is "What I have Eaten" minus "What I burned off Exercising".

So if you eat 1200, and sit around all day, you net 1200.

If you eat 1200, go to the gym, and burn off 300, you net 900. Because you ate 1200, but burned 300 of that off, so really, you're left with 900 calories for the day to fuel every other thing your body needs to do (keep you alive, prep meals, work, get groceries, etc).

You want a minimum of 1200 to fuel your body. Then more to fuel your exercise, so that you NET 1200 or more. MFP is asking you to net your target by eating back your exercise calories.

*However*: Exercise burns, no matter how you get them -- HRM, MFP, machine at the gym, online calculator -- are estimates. And it is not uncommon for the estimates to be high, sometimes wildly so. (50% or more!) So many people start with eating back SOME and see how that goes. (50-75%) -- In say, 6 weeks, you should have enough data to see if you're losing at the expected rate, slower, or faster. And then you can adjust from there, how much of those exercise calories you eat.

I would also say that it is more important to eat them back the lower your target. If MFP says you can eat like 2000 calories before exercise, and you workout but aren't hungry enough to eat them back, I'd not be too worried. If you're regularly eating like 1200, and working out, *even* if you're not hungry, I'd advise making an effort to plan to get those calories in. Use calorie dense foods (small foods with lots of calories, like oil, peanut butter, full fat dairy, etc) to boost the calories without boosting the volume of food (so you aren't eating a ton MORE, just higher cal).