A stance is a general combat strategy that you enter by using an action with the stance trait, and you remain in for some time. A stance lasts until you get knocked out, until its requirements (if any) are violated, until the encounter ends, or until you use a stance action again, whichever comes first. After you take an action with the stance trait, you can’t take another one for 1 round. You can enter or be in a stance only in encounter mode. You can Dismiss a stance.
Source: Player Core pg. 149
Requirements: You are wielding a shield
Once you’ve had a moment to set your stance, you always have your shield ready without a thought. While you are in this stance, you constantly have your shield raised as if you’d used the Raise a Shield action.