What does it mean to "eat at a deficit"????

less than needed to maintain your current weight, that is if you want to lose weight.

Your body needs a certain amount of calories to maintain its current weight - this is your TDEE (total daily energy expenditure). To eat at a deficit means to eat a quantity of calories below your TDEE. In order to lose weight, you must eat at a deficit. If you eat above your TDEE, you will gain weight.

Woohoo!!! Thank you!!! A simple sentence that I understand!

So if I need 2000 calories to maintain this weight then I need to have less than that to lose weight? Is that right?

Exactly....but too big of a deficit can also be counterproductive to your goals. If your deficit is too large you lose more LBM and muscle than you otherwise would.

The calorie goal MfP gives you has a built in deficit from a maintenance level of calories...MFP's goal is NET of exercise which is why MfP has you eat back exercise calories to avoid being at too large a deficit.

Thank you. What is LBM though???

I see what you mean about eating back exercise calories - I haven't been eating mine thinking it would help me lose weight but I understand now that it doesn't. Thank you!

Lean body mass. Usually that refers to muscle. You don't want to lose that.