a villain is essentially a timed mission for everyone in the server to attack together, each attack takes 3 minutes, if you have a 50% mission booster, or 4 or 5 minutes if you dont,i cant recall exactly , the villain can appear at any time and at any location,so as a low level you might not always see him to attack since you wont have all zones unlocked , he will appear with a hp(health points) and a time limit, the goal for the server is to attack him enough times that his health runs out before the time runs out , the hp and the time will vary each time , your goal as an individual would most likely to try and be in the top 10 attacks, so have attacked him the most (ranked 1-10) ,or to get the final hit . these offer the best rewards, and offer a higher chance of the suit items . If his time runs out before his hp then the villain will flee , giving no items but a bit of xp and coins instead ., where as completing it gives items, the chance of extra training and extra energy to use as well as xp and coins .
I hope that is clear enough for you