Explain Calorie Deficit

Your body uses a certain number of calories every day. This includes the calories it uses keeping your body alive, everyday activities like brushing your teeth, and the calories you burn through exercise. This number is your TDEE (total daily energy expenditure). Your deficit is the number of calories below that number that you eat.

You can get an estimate of your TDEE with any of the calculators floating around on the Internet (here's one: http://iifym.com/tdee-calculator/ ) but be aware that it's only a starting point. No one fits the equations exactly, so you might need a little trial and error to find your sweet spot.