When something goes "boom" and shoots bullets, you have to figure out what happens when the bullets hit something. That's what terminal ballistics is all about.
Now, imagine you're shooting a water balloon with a toy gun. When the bullet hits the balloon, the balloon pops and the water goes everywhere. That's what happens when bullets hit things too - they create a lot of damage.
But some bullets create more damage than others. If you shoot a big bullet at the water balloon, it will make a big hole and the water will gush out faster. If you shoot a small bullet, it might make a small hole and the water will trickle out.
Terminal ballistics is all about figuring out how a bullet will damage something when it hits it. Scientists and doctors study this so they can figure out how to make better bullets that can do more damage (like for hunting or protecting people) or how to make things that can stop the bullets from doing too much damage (like bulletproof vests).