A calorie deficit means that you consume fewer calories than your body needs to function. It helps in weight loss because your body starts burning stored fat for energy. Full definition
Or eat in calorie surplus for one week and then eat in calorie deficit for one week etc.. (roughstrength.com)
You need to watch your food and create calorie deficit in order to burn fat. (women-workout-routines.com)
The most common guideline for calorie deficits for fat loss is to reduce your calories by at least 500, but not more than 1000 below your maintenance level. (bodybuildingforyou.com)