Jet lag is the physical result of traveling from one time zone to another. If you wake up in Hawaii and try to fall asleep in Kentucky, you'll probably experience jet lag.
After flying to China, your jet lag might make you feel irritable and fatigued, and it may be impossible to fall asleep at night. These symptoms can last for several days, and they're caused by the sleep/wake rhythms of your body being disturbed. The condition didn't exist until the invention of jet planes, which allowed people to travel long distances very quickly. Originally, it was called "time zone syndrome," which was replaced by jet lag in the 1960s.