A mounting block is like a step-stool for humans! When you want to ride a horse or a pony, you have to get up high to sit on its back. But it's hard to get up there all by yourself, especially if you're not very tall.
That's why people use mounting blocks - they make it easier to get on the horse's back! A mounting block is like a big block that you step on. It's not too high off the ground, but it's high enough that you can climb onto the horse's back without having to jump or strain yourself.
It's important to use a mounting block because it's safer for both you and the horse. If you try to get on the horse's back without one, you might accidentally hurt the horse by pulling on its mane or back, or you could hurt yourself by falling off. But with a mounting block, you can climb up gently and easily, with no risk of hurting anyone or anything.