This is one of the four lessons you learn in the academy. It teaches you about how zones and pvp work. There are three types of zones in-game(safe, danger, and war). The symbol for each is located to down below.
Safe Zone - Players of the same village can online fight one another if both activate PvP mode. Enemy village ninjas will always be able to fight in any zone, including safe zones.
Danger Zone - A ninja of the same village may be attacked if either one has turned on PvP mode. In other words, turning on PvP mode means you will be able to attack any ninja of the same village, whether or not they have PvP mode turned on, but it also makes you vulnerable to attacks from all ninjas of your village.
War Zone - Same village ninjas can attack one another freely. PvP mode is always active in a war zone.
**KILLING FELLOW VILLAGERS IS OUTLAWED; any person caught doing so will be punished**