An attribute is a knack, perk, or aptitude that a hero has, one that grants them certain benefits in play. Each attribute has one or more levels of increasingly-strong effect. The benefits of an attribute stack, so if you take level 2 in one, you get all the benefits from level 1 as well.
Any character can pick any attribute, and it doesn’t have to “match” the class you selected. The only obligation is that you explain how your character picked up the training or exhibited their natural aptitude.
Unless indicated otherwise an attribute can be taken only once. Some skill-oriented attributes allow you to pick them more than once to apply to different skills.
Beginning PCs can pick one level of attribute. PCs with the Expert class or the Partial Expert Adventurer class option can choose an additional level in any non-combat attribute related to their background, including level 2 in their initial choice. PCs with the Warrior class or the Partial Warrior Adventurer class option can do the same in choosing an additional combat-related attribute.
As your hero advances in experience, you’ll get the opportunity to pick up new attribute or improve existing ones as you refine your talents or learn new tricks.