Imagine a bunch of marbles in a cup, all together they make a big, heavy cup. Now imagine putting too many marbles in the cup, it becomes too full and starts spilling over.
The same thing can happen with nuclear weapons. Countries all over the world have nuclear weapons which can be used to attack other countries. Now imagine if every country starts making more and more nuclear weapons. It becomes like putting too many marbles in the cup, and eventually, the cup spills over - this is called the "nuclear tipping point."
The tipping point is when there are so many nuclear weapons around the world that even a small mistake or misunderstanding can cause a big disaster. If so many countries have nuclear weapons, it becomes more and more dangerous, and an accident or misunderstanding could cause war or any other disaster. We need to be careful about how we use nuclear weapons and learn to live in peace with each other.