Your calorie deficit is the amount of calories that you eat minus the amount of calories that you burn in one day. For example if you eat 4000 calories but burn 4500 calories in the same day then you have a calorie deficit of 500 calories. A calorie deficit is required for any weight loss.
Most people find that it is easier to cut out foods from their diet and eat less than exercising more. This is easy to understand because a single pop-tart is equivalent to about a half hour on the treadmill. Given the choice most would find it easier to not eat a pop-tart rather than spending a grueling 30 minutes in the gym.