Air supremacy means that one side in a war or conflict has complete control of the skies. It means that their airplanes and other flying machines can fly where they want, when they want, and do whatever they want without fear of being attacked.
Think of it as playing a game of catch with your friend. If your friend can't catch the ball, you have the upper hand and can throw as many balls as you want without fear of your friend catching them. If you have air supremacy, you can fly as many planes as you want without fear of being attacked.
Having air supremacy can give a big advantage to one country during a war. They can use their planes to bomb and destroy enemy targets, and prevent the other side from using their own airplanes. It also means that the enemy's ground forces can be seen from the sky, making it easier to attack and defeat them.
To sum up, air supremacy is when one side has complete control of the skies and can use their airplanes to attack the enemy and defend their own ground forces.