Okay kiddo, so the United States and Japan have decided to work together on something called a "nuclear energy action plan." This means they're going to figure out how to use nuclear power in a safe and responsible way.
Nuclear power is kind of like a big battery that makes electricity. But it's different from the batteries in our toys or phones because it uses tiny things called atoms to create energy. Sometimes, if we don't use nuclear power safely, it can be very dangerous or even cause big explosions, like what happened a long time ago in a place called Chernobyl.
So the United States and Japan are going to work together to make sure that if we use nuclear power, we do it in the best and safest way possible. They are going to share their knowledge and technology with each other, so they can make better plans for using nuclear power in a way that won't hurt anyone.
It's like if you and your friend are playing with toys, but you want to make sure you don't hurt yourselves or anyone else. So you talk to each other and figure out the best way to share your toys and play safely. That's what the United States and Japan are doing- they're just working together to be safer and smarter when it comes to this kind of energy.