In more detail
Your body uses a certain amount of energy each day, called your TDEE. If the calories you eat are lower than that figure, the difference is the deficit. A common rule of thumb says roughly 7,700 kcal corresponds to 1 kg of body fat, so a deficit of 500 kcal a day, which is 3,500 kcal a week, suggests about 0.45 kg a week. The real rate slows over time as the body adapts and weight falls.
Why it matters
Knowing your maintenance level lets you pick a deficit that is large enough to see progress but small enough to sustain. Very large deficits can cause muscle loss, fatigue and nutrient shortfalls, and eating below your BMR (basal metabolic rate) is not generally recommended without medical supervision.
Example
If your TDEE is 2,720 kcal and you eat 2,220 kcal a day, your deficit is 500 kcal. The Calorie Deficit Calculator estimates how long a target loss would take.
Common confusion
A calorie deficit does not say what kind of weight is lost, and the 7,700 kcal rule is an approximation, not a law. Tracking real weight over several weeks is a better guide than any formula.