What is nuclear binding energy?
Nuclear binding energy holds the particles of the nucleus of an atom together.
Consider that the average nucleus of an atom consists of one or more protons and in most cases a complimentary or reasonably equal number of neutrons.
Consider also that if you were to concentrate two or more or many more positively charged particles into a tiny space that they would not appreciate it because like charges repel each other.
Fortunately there are neutrons present that space the protons apart slightly, but there still remains the repulsive forces. The nuclear binding energy counter-acts these forces to keep the atom together.
Looking at the atom from its outside, the nuclear binding energy would be defined as the amount of energy need to smash that atom into its component particles.