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 it is fleeing it is moved directly away from the charging enemy by 2D6" if its movement characteristic is 6" or less, or 3D6" if its move is more than 6'. The fleeing unit is repositioned facing away from the chargers. Move the fleeing models immediately the distance indicated by the dice roll. This is explained in more detail in the full rules governing fleeing models in the Close Combat section.