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. 147
The slightest distraction can provoke your wrath, and you’re prepared to foil enemies’ actions. As long as you are in this stance, you can use Reactive Strike when a creature within your reach uses a concentrate action, in addition to manipulate and move actions. Furthermore, you disrupt a triggering concentrate or manipulate action if your Strike hits (not only if it’s a critical hit).