Traits
Traits are special properties of characters that modify their existing statistics or grant them wholly new abilities, usually on the basis of a character meeting certain prerequisites to use them. The most common sort of traits are heritage traits (in settings that use heritages), which are taken during character generation. Characters continue to accumulate traits as part of landmark advancement at a set rate, usually one new trait per level.
Trait Prerequisites
Traits almost always have a prerequisite that a character must meet before they are able to take the trait. For example, the stock system trait Leaps of Logic (Trait) requires the user meet a certain minimum score for both Analysis and Intuition to qualify for the trait effects.
If a user meets the requirements for a trait after applying all other relevant changes during landmark advancement, the character may take the trait. If some future change to the character drives them under the requirements for the trait, the trait is no longer active, until the necessary aptitude or skill changes recover.
Trait Effects
Traits by and large have two classes of effects: passive, and active, of which the former is by far more common. Regardless of whether or not the trait is active or passive, it may also have a drawback. The drawback is never optional, but may be conditional. See the specific description of the trait you're interested in for details.
Passive Effects
Passive trait effects are those effects which are always active and which require no input from the character or offer any choice about their activation. For example, the Howl Basin-specific heritage trait Jack Lagos causes a character with this trait to recover Willpower-based pools at double the normal rate. The character does not need to decide to have this happen or pay a cost (other than the cost of having chosen it as one of their limited trait choices) for this to happen; it simply happens.
Active Effects (Abilities)
An active trait is any trait where the activation of the trait effect is optional. Using the same Leaps of Logic (trait) stock trait as an example, the character which has this trait may choose to pass an Intuition check as part of this challenge, which grants them a boon to the corresponding Analysis-based check if equally successful.
A distinction here should be made between normal traits and abilities. An Ability is effectively an Active Trait, but one which incurs a cost rather than simply having a drawback. This cost is almost always the deduction of points from a pool when activating the ability. The stock system for magic treats individual spells as individual abilities. A hypothetical "barbarian rage" ability might deplete some hypothetical "rage" pool, based on Strength or Hardiness or even Willpower. The distinction between abilities and traits is largely semantic.
Retraining Traits
By default, most traits are considered immutable and cannot be changed. In long-running campaign gameplay, Facilitators are encouraged to consider allowing players to retrain a trait, perhaps replacing it during extended downtime or as part of landmark advancement