An expression said by a man to another man. Can be said also by a woman to a man, but that is much more rare and weird.
Simply, the expression "You've got no balls" can stand for:
You have no self-respect, you are a coward, you can't face your problems like a man, etc.

After the expression is told to someone, he has to prove otherwise- do what he was told to do, or simply say:
"I do have balls. You wanna see them?"