Every troop type has a different carrying capacity or Load value. The higher the unit's Load value, the more resources it can carry. All your troop's Load values are added together to represent the army's total Load. 

 
Troops on an expedition will consume Food. The farther they travel, the more Food they will require. 

 
If the army's Load isn't sufficient for the distance you plan on traveling, you will be unable to go on that expedition. 

 
If the army's Load is insufficient, there are several things you can do: dispatch your army on a shorter expedition, reduce Food consumption, or add more troops with higher Load values to your ranks. 

 
In addition, when plundering, your troops will not be able to plunder any more than the army's total Load value. 

 
Upgrading your Logistics level at the Academy will increase your army's Load.