Look up TDEE and you’ll be able to find a calculator, it’ll tell you what calories you need to just to live, you’ll work out if you’re sedentary/active and what calories you should burn with that, plus how much to eat so it becomes less than what you burn-up and this equals your deficit. Does that make sense? It should work out that you burn up more in being alive/working out than what you eat.
Calorie deficit is simply eating less calories than you need to maintain your weight. Just like deficit spending is.. spending more than you earn, a calorie deficit is spending more calories than you earn.