a trackbar is a 5th suspension arm that holds the axle from moving laterally (side to side). it goes from driverside of the jeep to passanger side of the axle in the front, and oposite in the back.
if you can visualize what happens when you lift your jeep 3.5 inches, you can see how the axle gets pulled a few inches to the driverside (in the front) if you dont adjust the trackbar.
the IRO trackbar conversion adds a longer adjustable track bar, and a new bracket and joint that is significantely stronger, and allows more rotation than the stock one.
in my opinion, i would still add a rear track bar on top of that kit too. and control arms as well