Following Guest:Ripov's comment:
Flashing headlights is now usually an "invitation to proceed" to another driver, but needs to be interpreted carefully.
If visiting the UK that is a very dangerous piece of advice. According to the Highway Code the only meaning of flashed headlights is to let another driver know that you are there.
Yes, it is, sometimes, used to gesture to let another driver out, but this cannot be relied on. Similarly, lorry drivers being overtaken will often flash headlights to let a passing vehicle know that it has passed far enough to be able to pull back in. But that is courtesy only. It has no legal backing.