Flee means just that - when your unit sees the enemy thundering down upon them, they turn tail and run! This is a rather desperate option as once troops begin to run they tend to carry on going, and may run away from the battlefield altogether, whether you want them to or not.

As soon as a unit declares that it is fleeing, it is moved directly away from charging enemies by 2D6" if its Movement characteristic is 6" or less, or by 3D6" if its Move is more than 6". The fleeing unit is repositioned facing directly away from the chargers. Move the fleeing models immediately by the distance indicated by the dice roll. This is explained further in the rules for fleeing models in the Close Combat section.