A jiffy is a brief amount of time — an instant, or a split second. When you're really tired at the end of a long day, you probably fall asleep in a jiffy after lying in bed.
Jiffy is an informal word that can substitute for "quickly" or "in the blink of an eye." A speeding taxi will get you to your destination in a jiffy, and a really well-trained dog will come in a jiffy when you call her name. Not much is known about the origin of jiffy, other than the fact that it was once "thieves' slang" for "lightning."