You burn a lot of calories simply by being alive.

The more active you are the more you burn.

You can think of it like fuels consumption for a car.

You engine burns fuel even if you are just idling, fuel consumption goes up with driving faster or making the engine work harder in other ways.

You goal is to pump less extra "fuel" into your mouth than you burn each day.

Exercising and making you body work harder in other ways helps with that.