It means what it sounds like: you were warned that you were driving recklessly, or potentially dangerously and you need to drive more carefully or that you might face consequences next time--but you are not facing any consequences now. For whatever reason--maybe your driving was somewhat reckless but not enough so that the officer felt you met the criteria for a ticket; maybe the officer was in a generous mood and/or you seemed receptive to a warning--he decided to not punish you this time, but did warn you that you need to do better. With no ticket, you should not face any consequences from this stop.