Some signals are pretty straightforward: Turn signals, when people can be bothered to use them are a good example. You can be pretty certain that the car ahead of you will turn right or pull over if their right turn signal comes on. If it is on all the time, then all bets are off; it is more of a documentation that they did in fact signal on their last right turn.
Flashing the high beams can be done for a lot of reasons, so it is context dependent. It might be a signal that there is some danger ahead, or you left your high beams on dummy! A brief flash can convey a friendly nudge, sustained or repeated flashing or sustained high beam is a Hey Buddy Get Your Head Out of the Cloud! Most of the time the high beam is a way of saying, "I am welcoming you in ahead of me". Say you are in stop and go traffic and someone wants in or wants to turn left across traffic. Unless someone lets them in, they will be stuck all day. So if you are in the right position to help, flash your lights and let them through.
So, the case at hand: I was traveling down a main suburban road at the speed limit of 30. The road is lightly traveled even on weekday mornings. A car approached from the opposite direction and stopped, I don't think she had her turn signal on but I have seen that car in the driveway of the house I was passing, so I figured she wanted to go there. She flashed her high beams as I approached. What is she trying to tell me? I think I know, but it is so crazy that I have a hard time believing it. First of all: The person doing the flashing is the one making an offer, that is the rule. The person trying to make a left or get on the road doesn't flash his high beams--we all know what he wants. The person giving up his right of way is the one doing the flashing, as a way of signaling his intentions and generosity.
Did this lady really signal me to stop so that she could make a left turn on this pretty-much empty road? Yes, I think that was her intent. She must come home from every drive disappointed with the Human Race for not all getting the hell out of her majesty's way.