Traits represent any being's most basic and inherent capabilities: how strong are they? How intelligent?
Traits have a wide variety of effects on the game. First, some abilities can only be selected by characters who meet a certain trait requirement. Second, traits are used in many checks within combat: attacking, defending, using magic successfully, maintaining positive conditions and recovering from negative ones, and so on. Third, some equipment cannot be equipped unless you have the necessary score in a trait. Fourth, some vote options are only available to parties with characters that have a sufficiently high score in a certain trait. Finally, other scores in the game derive from certain traits: for example, your Health score depends partly on your Fortitude.