Alright kiddo, after the big war ended, many countries had airplanes and they used them to fight in battles. Sometimes, these airplanes would get shot down by the enemy's airplanes, and this is what we call an air-to-air combat loss.
Basically, imagine if you and your friend were playing a game of tag, but instead of running around on the ground, you were flying in the sky on cool airplanes.
Now sometimes, your friend would tag you and you'd have to go back to base. In air-to-air combat, if an enemy's airplane tags your friend's airplane, that airplane has to go back to base, but sometimes it means the airplane is destroyed and won't be able to fly anymore.
After World War II ended, a lot of countries were still having these air-to-air battles, and unfortunately, a lot of airplanes were getting shot down or destroyed. People studied why this was happening and came up with different ways to make airplanes better and safer so that they wouldn't get shot down as easily.
And that's the story of post-World War II air-to-air combat losses, kiddo.