The Short Answer
Calories are a measure of energy.
Your daily calorie needs depend on factors including body size, age, sex, activity and goals.
Maintenance
Eating around your maintenance level tends to maintain body weight over time, although actual weight fluctuates.
Calorie Deficit
Consistently consuming fewer calories than you use tends to produce weight loss.
Calorie Surplus
Consistently consuming more energy than you use tends to support weight gain.
Why Does Everyone Need a Different Number?
A 50 kg person and a 100 kg person do not generally have identical energy needs.
Activity also matters.
Your calorie needs can change as your body weight and activity change.
Use the Praveen Trains Calorie Calculator and TDEE Calculator as starting points rather than absolute truths.