What is the strong nuclear force of an atom?
The strong nuclear force holds protons and neutrons together in the nucleus.
The nucleus of an atom shouldn't really stick together, because protons and protons have the same charge so repel each other. It's like putting two North ends of a magnet together - it doesn't work.
But it does, because of the strong force, so-called because it's strong. It holds the two like-ends of the magnet together, and so keeps the whole atom from falling apart. The boson (force particle) of the strong force is called a gluon, because it is basically a glue.
When the nucleus is unbalanced, when it has too many protons or too many neutrons, the strong force isn't quite strong enough, and so the nucleus loses protons and neutrons (