Well, imagine that you're playing with some toys and suddenly another kid comes along and wants to take your toys away. You might start to feel scared or worried that they will hurt you or take all of your toys.
Now let's take that same scenario and apply it to countries in the world. When one country sees another country becoming more powerful or maybe doing some things that they don't like, they might start to feel threatened just like you did with your toys.
So, the balance of threat is basically when a country tries to balance out that fear or threat from another country by building up their own military or alliances with other countries. It's kind of like when you ask your friends to come play with you so that the other kid won't take all of your toys.
Countries want to make sure that they are strong enough to defend themselves and their people, but they also don't want to be too aggressive or start a war. So, they try to find a way to balance out the threat from other countries while still maintaining peace and stability in the world.