It's actually quite simple and it involves the food you eat and how much energy your body uses throughout the day. A calorie is a simple measurement of the stored energy within a given food that we can consume.

1 calorie in food is how much energy it takes to raise one kilogram of water one degree Celsius when burned in a controlled environment. It's an imperfect science because there are things like fiber that will burn in this setup but do not confer energy to a human body.

Your body needs a set amount of energy (calories) to operate throughout the day depending on your muscle mass, body size, gender/sex, and activity. This is your baseline calorie need for a given 24 hour period.

Now, you eat food throughout the day and that has a caloric value. If you eat less calories than your body uses then you have a calorie deficit for the day and assuming you have eaten healthy and your body's endocannabinoid system (pancreas, thyroid, etc.) is healthy then you will either consume fat stores or muscle mass to maintain necessary bodily functions. If there are insufficient fat or muscle stores in your body then you start to consume major organs until death occurs.

TL:DR eat less than you require and you're at a calorie deficit. To bolster weight loss, gain muscle mass as that is the most energy hungry thing you can control the amount of in your body.