Characters set to secure mode cannot attack other characters that are not marked by a skull. This way it is not possible to attack innocent bystanders or friends by accident. Note that you can always defend yourself against attacks in this mode because aggressors will be marked automatically. Also, keep in mind that the secure mode will not prevent that your character may get a skull mark if he causes damage, e.g. by fire fields or explosions.
Your character is in secure mode by default. If you want to switch to normal mode, click on the button with the white hand symbol to the right of your inventory to turn it into a fist symbol. In this mode you can freely attack all other characters. Keep in mind, however, that your character will receive a skull mark if you attack unmarked characters.