A calorie deficit is when the number of calories you take in is less than the calories you burn in a given period, generally a day.

If you eat 1800 calories, to achieve a deficit you must burn more than that. For instance, I heard a long time ago that a single jumping jack burns 1/2 a calorie.

There is a thing called resting metabolic rate, which tells you the number of calories burned just staying alive in a day. This will be helpful to know, you can consider it the "minimum burned calories" in the sense that everything you do in a day burns additional calories. (E.g. I'm 6'1", 200lb 27 year old male, mine is 1800-2100 depending on the rough calculation)

Everything you eat counts against calories burned. If you want to stay in a deficit, you need to balance calories with exercise. You do not need to do a 10,000 calorie exercise to justify a 500 calorie treat, but maybe monitor your other choices that day.

It is important to note that you can achieve calorie deficit without exercise, but also the effects of exercise are wonderful and it allows you to "live a little" with food choices while trying to maintain a caloric deficit.

Best of luck on your journey.