Dog's don't instinctively know what any command means. So don't assume your dog knows the command, you must teach it to him. Of course there are people who train their dog and he still won't come. Because many people in their attempt to teach their dog to come, actually teach their dog not to come. First of all, let's look at how dogs learn.
Dogs learn everything by association.
When teaching a command, we give a verbal command or hand signal, show the dog what we want, and when he obeys we reward him. The dog then associates the command with a certain action for which he is rewarded. The dog learns through this association what is expected of him. The dog obeys the command because of the positive reinforcement, or praise that he receives for his action. The mistake that many owners make is that they teach their dog to associate coming to them with something negative.