A jiffie is a variable measure of time, for a woman it is a matter of seconds and for a man it is a matter of hours. For example, my wife will say "I'll just rush and and talk to my friend, I'll be back in just a jiffie."
Oh, you want meaningful information. Guess maybe Dave could answer that, I notice the value changes each time I load the page, so maybe it is a measure of the time it takes the page to load.
Seedfork is exactly right. A jiffy is 1% of a second (meaning, it takes 100 jiffies to make a second) and the number appears because it's telling you how long it took the server to generate that page.
ATP is actually a software program. Each page is created automatically by the software and it takes computational resources to generate those pages. Particularly heavy pages get the timer so I can monitor if a page is taking too long to load.