A calorie deficit means you take in less energy than you burn over a longer period. Your body covers the rest from its stores, mainly fat tissue. Without a deficit there is no fat loss, whatever diet you follow.
How big the deficit should be
The bigger the deficit, the faster your weight drops, but the higher the risk of losing muscle and training performance. For lifters, losing about 0.5 to 1 percent of body weight per week is considered a good range (Helms et al. 2014). The leaner you are, the more the lower end fits.
A meta-analysis by Murphy and Koehler (2022) found that lean mass gains were smaller in a deficit. From about 500 kcal per day, lean mass gains no longer occurred. Strength, on the other hand, hardly suffered.
Example: At 80 kg and 0.5 percent per week, that is 0.40 kg per week. Using the approximation of 7,700 kcal per kilogram, this gives 0.40 × 7,700 ÷ 7 = about 440 kcal deficit per day. These numbers are a worked example.
The deficit shifts over time
As your weight drops, so does your energy expenditure, and many people move less without noticing. The same food gives a smaller deficit after a few weeks. That is why you keep checking your rate against your trend weight and adjust calories in small steps after two to three weeks if the trend is clearly off target.
Besides the size of the deficit, protein and training matter: plenty of protein and continued heavy sets give your body a reason to keep its muscle.