Evasion is a defensive stat enabling you to stop damage from happening to your character.
Evasion is affected by the amount given by the base stance, how much additional is added as the stance progresses, agility and gear.
Agility affects block as a percentage of the amount granted by the stance. For every point of agility above 5 you gain 10% of the base stance amout. [Unconfirmed: every point below 5 should drop the amount by 10%.] If the stance grants a total of 30, then 8 agility (+3 over 5) will increase that by 30% to 39 before adding gear.
Body armor and shoes can both provide evasion mods.
Evasion cannot protect your character against ranged physical attacks (eg: guns). [Unconfirmed: Evasion is supposedly penalized by the number of simultaneous attackers engaged with you.]