Jackknifing occurs when the trailer of a big rig or other articulated vehicle loses traction and skids sideways, forming an L or V shape with the tow vehicle. Here is an explanation of how to prevent it.
Follow the manufacturer's recommendations for loading capacity of trailer and tow vehicle. Never drive with an overloaded or unbalanced trailer.
Invest in anti-jackknife systems for your tow vehicle and trailer. Modern antilock trailer braking systems can sense tire skid and automatically adjust braking force to prevent lockup.