What Does Flashing Headlights Mean?
On Road
If you are driving a vehicle on road, there could be several meanings of flashing headlights.

To Warn Others of a Police Presence. You might have noticed another vehicle, driving in a completely different lane is flashing at you. If you find this confusing, the other driver might be just trying to let you know that there is a police trap ahead.learn more

Sometimes, they even want you to know that there are cameras installed ahead, in case you are driving roughly or your headlights are damaged.

While it is not necessary to check your vehicle amid driving, you should definitely be careful with your speed if someone is flashing at you from a different lane. It might simply save you from the trouble. However, it is illegal in some states to flash headlights to warn about speed limits or police trap ahead.

Overtaking Vehicle. If a vehicle is flashing headlights from behind, then they might want to overtake your vehicle and indicate you do not increase the speed or take an extreme left or right, wherever they are coming from.

Communicating with Pedestrian or Cyclists. Sometimes rather than honking at someone (which is considered rude in some countries), many drivers prefer flashing headlights at cyclists or pedestrians to keep them safe.

Warning Of Hazards. It could be simply a warning of some ongoing work or bad weather conditions. Sometimes a vehicle behind you might be flashing the light after noticing that you haven’t reduced the vehicle’s speed where it was required.

It is recommended to slow down your vehicle a little whenever someone is flashing you from behind. Also, someone in the opposite direction might also flash their headlights to inform you about the hazards ahead.

Greeting. If a driver spots a friend and wants to grab their attention on the street, they may flash their headlights. Or, when your friend is trying to look for your vehicle on the street, you may want to simply flash the headlights, which makes it easier for them to locate you.

On Highway
On the highway, there is only one reason to flash headlights: You should let others know that you are also there on the highway. That's it.

According to the Highway Code: “Only flash your headlights to let other road users know that you are there. Do not flash your headlights to convey any other message or intimidate other road users.”

It means you cannot use your headlights for greeting, convey any message, or intimidate other drivers. If you misused the flashing headlights on the highway, then it can land you into trouble with the law.