When a semitruck "jackknifes," that means that it essentially folds sideways at the hinge between the truck and the trailer. They form a "V" shape that looks like a folding knife being closed -- thus the name.
This is a very dangerous situation. A truck that jackknifes is likely completely out of control. The driver can no longer steer properly. In addition, even on a wide, multilane road, the truck may take up all of the lanes. Everyone around the vehicle is in tremendous danger, and it may be impossible for them to avoid a crash.
Why does this happen? There are two main ways. The first is when the driver is on a slick road and does not adjust their driving properly to account for it. While the cab may maintain traction, if the trailer loses traction and begins to slide, it's hard to bring it back under control. The trailer also holds most of the vehicle's weight, so it can quickly push the cab out of control, as well.
The second way that this happens is when the driver brakes improperly. Maybe he or she did not see traffic stopped ahead and had to slam on the brakes at the last second. This can lock up the wheels, making them slide on the pavement, and the heavy trailer may begin to swing around to the side. This is just one reason drivers always need to be aware of what is happening ahead of them.
If you get injured in an accident with a semitruck, you may be able to seek compensation for your medical bills.