According to the Highway Code, being flashed by another driver means that they are alerting you to their presence. Informally, it could mean a number of things, but always be cautious. Apart from courteous gestures that may involve headlight flashing, if you receive random flashes from other drivers it could signify a problem. You may be driving with full beam headlights and dazzling other road users, so check if the blue full beam light is illuminated and if so switch to dipped lights. Alternatively, you may have your fog lights turned on when the conditions are not foggy and therefore dazzling others. If it’s getting dark you may have forgotten to put your lights on, or a front (or rear if flashed from behind) light may have failed, so check it out.