Traits are used in both the Rome: Total War and Medieval II: Total War games to control character attributes. For example, how good a general is at attacking in a siege battle, how likely he is to have children; or, for a spy, how successful he is likely to be in carrying out his missions.
Traits are assigned according to triggers that fire under certain circumstances, eg. when a general wins a battle, or stays in a settlement that has a certain building, or when a spy successfully completes a mission.
Guides to Traits in Gameplay: