You're driving down the interstate at night when a semi-truck passes you and another car. As it goes by, you notice the other car flash its headlights at the truck. It's just a quick flash, perhaps two in quick succession. The truck then flashes back with its taillights and changes lanes.
What just happened?
This is a common signal for truck drivers. It just means that the lane is clear. It gives the trucker permission to switch lanes without causing an accident.
What the driver ahead of you knows is that the truck driver, in such a massive vehicle, may have a bit of trouble knowing when the truck is completely around the cars. If he waits too long to switch lanes, he blocks the entire highway, which can lead to traffic congestion and accidents. The other driver is just letting the truck driver know that he or she is clear and can move over safely.
The problem is that this is driver shorthand, not a written rule. Things like this can lead to miscommunication, which is dangerous.
For instance, suppose that you thought you should flash your lights while passing a truck to let the driver know you were there. The driver may take that as permission to change lanes, while you would think you were expressly telling them not to change lanes. The moment of confusion could lead to a serious wreck.
If you get injured in a truck accident, whether it involves this type of signaling or not, make sure you know if you have a right to financial compensation for medical bills and other costs.