This is the act of massaging the penis, with the intention of increasing its size (both length and girth). Average increase is said to be between 1 and 4 inches in length, and 1 to 2 inches in girth. It is also said to be good for increasing your libido, and can strengthen your ejaculations.
It probably originated in ancient Arabia, and is thought to have been taught to the young men before they got married. This would have been done at a young age, since marriage was usually effected between the ages of fourteen and seventeen.
The word “Jelqing” is believed to be of either Arabian descent, or a corruption of the English word “Jerk” as in “Jerk-Off”. However, if you care about this fact, you really shouldnt be reading this as you probably dont need a bigger penis.