Okay, so you know how sometimes things explode really big and create a lot of heat and fire? That's kind of like what a high explosive incendiary is, but it's a special kind of explosion that's supposed to start fires and burn things up.
The explosion itself is made up of a bunch of chemicals that burn really fast and create a lot of heat and pressure. When this explosion happens, it can ignite things that are nearby and start a fire. That's why it's called an incendiary, because it starts fires.
Now, the "high explosive" part of the name means that it's a really powerful explosion. This type of explosive can do a lot of damage and destroy things like buildings or vehicles.
Sometimes people use high explosive incendiaries for things like military operations or demolishing buildings. But it's important to be really careful when handling them, because they can be very dangerous.
So, in summary, a high explosive incendiary is an explosion that starts fires and burns things up, and it's really powerful and dangerous.