Brake balance is when drivers manipulate the brake pressure of the front and rear brakes by a switch in the cockpit. F1 cars need different brake power in front and rear brakes throughout the race and drivers can regulate it using this dial.
Usually, drivers use a 60:40 or 55:45 split and usually, the front brake gets more power. In the 2021 F1 season, the brake bias was 50% to 70%.
Too much front brakes will lock up the wheels and too much rear brakes will make the car unstable. In dry conditions, more brake bias is moved to the front whereas in a wet condition more brake bias is pushed to the rear.