If you mean the common truck-driver signaling convention, a brief single flash of headlights from the truck behind you after you overtake can mean “you can move back in front of me”—essentially, “You’re clear; thanks for passing.”
However, headlight flashing isn’t universal. Depending on the country, road, and driver, a flash can also mean “watch out,” “I’m letting you know I’m here,” or simply be an impatient gesture.
If you’re in Pakistan, don’t assume a single flash has a standardized legal meaning. The safest rule is to check your mirrors, leave a comfortable gap, and only return to the lane when you can clearly see enough of the truck in your mirror. Don’t move over solely because the driver flashed you.
