Trait is a characteristic of your avatar. It usually has a positive and a negative side to it.
For example, the trait Bruiser increases your Strength by 2 points, but also decreases your ActionPoints by 2.
You can choose up to two traits at the beginning of the game, during the CharacterCreation process.
List of currently implemented traits:
There are more traits in the original Fallout 2 game - they will most likely come with later game updates.