Bearded dragons wave for many different reasons, including showing larger bearded dragons that they are submissive, as a sign of recognition between bearded dragon individuals, a way to assert dominance and a way to intimidate. The wave is a common move among the species.
For the most part, the arm waving is considered a sign of submission. However when the wave is fast and the bearded dragon also moves its head then it may be a sign of dominance. Sometimes bearded dragons will be nervous or worried about another dragon that is walking nearby or by a human, so they start waving their arm slowly to try to intimidate the dragon or human and show their dominance. The true message of the bearded dragon's arm wave depends on context.