it is a very real condition that results from a prolonged state of sexual arousal. When a guy is physically turned on, blood flows to his penis, which is what gives him an erection, and his testicles, causing them to swell. If he doesn't ejaculate, there is a buildup of pressure, and his super sensitive balls feel the brunt of it. The sensation can range from a mild ache to worse-than-getting-kicked-in-the-crotch pain. But the bottom line is, it's not dangerous, and he can deal with it, whether that means giving himself a helping hand or just waiting it out. The blood will eventually drain, and any discomfort will disappear on its own.
In case you were wondering, it's called blue balls because the blood that's been in the scrotum for a while loses oxygen and can give the appearance of a bluish tinge -- although it sounds like something guys came up with to express just how sad they are that they can't come!
Google be your best friend :)