Disruption during movement.

[Mowgli]

Unfortunately, the manual for the Napoleonic/Civil War battle titles is not very helpfull at all.

As krmiller has wirtten:

If a unit suffers casualties due to enemy opportunity fire, it may need to conduct a morale test. The likelihood depends on a) the size (manpower) of the unit and b) the number of casualties suffered. The smaller the unit and the higher the casualties, the more likely the unit needs to undergo a morale check. If the unit fails the morale check, it becomes disrupted (and immediately loses a third of its movement budget). If it passes, nothing happens.

Tips:
To prevent morale checks from opportunity fire, you may screen your units with other units (units block lines of sight! no line of sight -> no opportunity fire) or with skirmishers (shooting through skirmishers reduces fire effectiveness by 20% (not cumulative)).
To increase your chances to pass the morale check, have a commander stacked with the unit.