Its meaning is always a mystery but according to Beric, it would be in any case a message from the Night King. Unless it was the Night King who left the message, but the Forest Children themselves. From what Jon finds in the cave at Dragonstone and explains to Daenerys, the symbol would have represented the union of the First Men and Forest Children to fight the White Walkers. Originally created by the Children of the Forest to defend themselves from the First Men, they would have escaped the control of their creators and forced them to ally themselves with their former enemies to repel them. Maybe the Children of the Forest are still trying to get a message across so that all the living together in order to defeat the Night King. But in this case, would there still be a relationship with the Targaryen? We just have to wait for the next episodes on OCS hoping to have an answer.