Traits are a personal version of skills. Unlike skills, which are learned via classes, traits are learned via a unit's personal development. There are also unique traits, traits that are unique to a character and cannot be changed. Most characters have two personal traits (one which is unit-based, one which is interest-based), though some characters (such as the Avatar) have three (in this case, a unit-based trait, an interest-based trait, and a chosen trait). All characters have four traits other than their unique trait, which are unlocked by leveling up (not ranking up) a unit (and reaching levels 10, 20, 35, and 50).
See list of traits for more information.