If you mean Guard mode, when it is on, the unit won't move to engage an enemy, but just stand ground attacking anything that comes into range. This is good for keeping your units in formation, but for attacking, it can result in taking higher casualties in say for example phalanx fighting with Sacred Band Infantry versus Silver Shield Pikemen, where pikemen have longer pikes than the SB infantry, so if the SB infantry is told to stand ground and not ordered to attack, a pikemen unit will easily win as the SB infantry will not even be able to reach the pikemen, but if guard mode is off, then the SB infantry will win (the same effect can be achieved with guard mode on and an order to attack).