Overmatch is like a game where you have different sizes of blocks that you can stack on top of each other. When a big block lands on a small block, it can break it easily because it's too strong for the little one to hold up.
Now, in real life, this happens with guns and armor. If you shoot a small tank with a big gun, the bullet can go right through it, because the tank doesn't have enough armor to stop it. This is called overmatch.
It's like a big kid hitting a little kid. The little kid can't stop the big kid from hitting them because they're not strong enough. And that's what happens when a big bullet hits a little bit of armor. It just goes straight through, like it's not even there.