Nuclear security is about keeping dangerous materials and things away from bad people who want to hurt others. Just like you lock your front door to keep strangers out, countries with nuclear weapons do a lot to keep their weapons safe and secure.
Nuclear weapons are made up of tiny things called atoms that are split apart, and when it happens, a lot of energy is released - this is what makes them so powerful. To keep them safe, there are a lot of rules in place!
For example, countries that have nuclear weapons agree to follow something called the Nuclear Non-Proliferation Treaty. This means they promise not to give or spread their weapons, and they have to let other countries come and check that they're doing what they said they would.
There's also something called the International Atomic Energy Agency (IAEA) - they're like the police for nuclear materials. They work with countries to make sure their nuclear programs are used for peaceful purposes, like creating energy for electricity, and not for building weapons.
There are a ton of rules and regulations in place to make sure nuclear weapons don't fall into the wrong hands. It's important to keep them safe so that we can all live in a peaceful world.