It will return all messages that were said since the agent's last turn. This will be an array with as many elements as there are agents. If an agent didn't say anything on its last turn, then its element will be null. If it said something, then the most recent message (a string) will be associated with its element. Note, this means every agent will have an opportunity to hear everything said by the other agents, but the percepts will not contain history information from more than one full cycle ago, regardless of whether or not agents say anything new.