Sources of dexterity:
Relics: Oddly smooth stone
Potions: Dexterity potion
Dexterity is a positive status effect indicated by a green shield under the health bar.
Each stat of dexterity increases the amount of Block gained from cards by 1.
Dexterity can also be decreased or negative in certain situations, decreasing the amount of Block instead.
NOTE: Despite the tooltip, Dexterity only applies to cards that directly increase your Block, which generally means it only works for Skills. For example, Metallicize will not be affected by Dexterity.