There are three main reasons truck drivers flash their lights at you on interstates. 1. To alert other drivers that police are nearby 2. To alert other drivers that road conditions are poor 3. As a way to say hello
The first reason, to alert other drivers that police are nearby, is probably the most common reason for flashing headlights. In this case, the truck driver is telling you that he just passed a police officer hiding in the bushes that you're about to drive by. When this happens, you should check your speed and slow down to make sure you don't get pulled over as well.
The second reason, to alert other drivers that road conditions are poor, is also fairly common. This usually happens when the road has become slick or slippery due to rain or snow, or if there's been an accident ahead of them. If this happens to you and you see the truck driver flashing their headlights at you, be careful and slow down!
The third reason truck drivers flash their headlights at you is as a way of saying hello. This is pretty rare though since most truck drivers don't know each other. If you see one doing this though, it means they've probably recognized your vehicle from seeing it on the road before and want to give a friendly