Okay kiddo, so high-explosive incendiary/armor-piercing ammunition is a type of bullet that is used in big guns.
Now, I know you probably don't know much about guns, but think of it like this: when you shoot a normal bullet, it goes through things like paper or wood. But when you shoot a high-explosive incendiary/armor-piercing bullet, it can go through really tough things like metal or thick walls.
These bullets have two main parts: the armor-piercing core and the incendiary/explosive part. The armor-piercing core is made of really strong materials like tungsten or depleted uranium, and it's pointed so it can penetrate through objects. The incendiary/explosive part is what makes it really powerful. It can explode and burn things, which makes it even more destructive.
These bullets are used in situations where you need to take out a really tough target, like a tank or a heavily fortified building. They are also used by the military in combat situations.
But remember, guns and ammunition can be really dangerous, so it's always best to stay safe and not play with them unless there's a real emergency.